Patent Publication Number: US-2009239028-A1

Title: Honeycomb structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to PCT Applications No. PCT/JP2008/055455 filed Mar. 24, 2008, PCT/JP2008/055456 filed Mar. 24, 2008, PCT/JP2008/055458 filed Mar. 24, 2008, and PCT/JP2008/055459 filed Mar. 24, 2008. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a honeycomb structure. 
     2. Discussion of the Background 
     In recent years, particulate matter (hereinafter, also referred to simply as particulate or PM) contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks, and construction machines have raised serious problems as contaminants harmful to the environment and the human body. 
     For this reason, various honeycomb structures, which are made of porous ceramics, have been proposed as filters that capture particulate in exhaust gases and purify the exhaust gases. 
     As a honeycomb structure of this kind, for example, a honeycomb structure has been proposed in which, after a plurality of rectangular pillar-shaped honeycomb fired bodies have been combined with one another with an adhesive layer interposed therebetween, the combined honeycomb fired body undergoes a cutting process to be formed into a predetermined shape to manufacture the honeycomb structure (for example, see WO01/23069A1). 
     Further, a honeycomb structure has been also proposed in which a plurality of honeycomb fired bodies, each of which is manufactured by preliminarily being extrusion-molded into a predetermined shape, are combined with one another with an adhesive layer interposed therebetween (for example, see JP-A 2004-154718). 
     On a cross section perpendicular to a longitudinal direction of these honeycomb structures, a honeycomb fired body having a rectangular shape in the cross section is located in the center portion of the honeycomb structure. Honeycomb fired bodies having a smaller cross-sectional area than that of the honeycomb fired bodies located in the center portion are located in the peripheral portion of the honeycomb structure. 
     Moreover, a honeycomb structure having another structure has been proposed in which, on a cross section perpendicular to a longitudinal direction thereof, a honeycomb fired body having a rectangular shape in the cross section is located in the center portion of the honeycomb structure, and a honeycomb fired body having a cross-sectional area larger than that of a honeycomb fired body located in the center portion are located in the peripheral portion of the honeycomb structure (for example, see WO04/96414A1). 
     The contents of WO01/23069A1, JP-A 2004-154718 and WO04/96414A1 are incorporated herein by reference in their entirety. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a honeycomb structure includes a plurality of honeycomb fired bodies that are combined with one another with an adhesive layer interposed therebetween. Each of the honeycomb fired bodies has cell walls extending along a longitudinal direction of the honeycomb structure to define cells. The honeycomb fired bodies include a center-portion honeycomb fired body located in a center portion and a peripheral-portion honeycomb fired body located in a peripheral portion in a cross section perpendicular to the longitudinal direction of the honeycomb structure. A shape of the center-portion honeycomb fired body is a substantially rectangular shape in the cross section. An area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in the cross section. A shape of the peripheral-portion honeycomb fired body is different from the shape of the center-portion honeycomb fired body in the cross section. An area of the peripheral-portion honeycomb fired body is at least about 0.9 times and at most about 1.3 times larger than the area of the center-portion honeycomb fired body in the cross section. 
     According to another aspect of the present invention, a honeycomb structure includes a ceramic block. In the ceramic block, a plurality of honeycomb fired bodies are combined with one another with an adhesive layer interposed therebetween and each of the honeycomb fired bodies has cell walls extending along a longitudinal direction of the honeycomb structure to define cells. A plurality of the honeycomb fired bodies include a center-portion honeycomb fired body located in a center portion of the ceramic block and a peripheral-portion honeycomb fired body forming a part of a peripheral side face of the ceramic block. An area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in a cross section perpendicular to the longitudinal direction. Provided that a figure, which is similar to a shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, a part of the peripheral-portion honeycomb fired body is located in the figure. 
     According to further aspect of the present invention, a honeycomb structure includes a plurality of honeycomb fired bodies that are combined with one another with an adhesive layer interposed therebetween. Each of the honeycomb fired bodies has cell walls extending along a longitudinal direction of the honeycomb structure to define cells. The honeycomb structure includes a peripheral portion forming a peripheral side face of the honeycomb structure; and a center portion having a substantially rectangular shape located at the inner side of the peripheral portion in a cross section perpendicular to the longitudinal direction of the honeycomb structure. The peripheral portion includes a plurality of peripheral-portion honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween. The center portion includes one center-portion honeycomb fired body or a plurality of center-portion honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween. The honeycomb structure includes at least one of the adhesive layers in the peripheral portion formed in a direction extending from a corner point of the center portion to the peripheral side face of the honeycomb structure in the cross section. The adhesive layer extending from the corner point of the center portion to the peripheral side face of the honeycomb structure forms an angle of at least about 40° and at most about 50° with at least one adhesive layer formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure. 
     According to the other aspect of the present invention, a honeycomb structure includes a ceramic block. In the ceramic block, a plurality of honeycomb fired bodies are combined with one another with an adhesive layer interposed therebetween, and each of the honeycomb fired bodies has cell walls extending along a longitudinal direction of the honeycomb structure to define cells. An area of the honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in a cross section perpendicular to the longitudinal direction. An area of the ceramic block is at least about 10000 mm 2  and at most about 55000 mm 2  in the cross section. A number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from a center of gravity of the ceramic block to a periphery of the ceramic block in the cross section is two or less in a case that the area of the ceramic block in the cross section is about 10000 mm 2  or more and less than 25000 mm 2 , three or less in a case that the area of the ceramic block in the cross section is 25000 mm 2  or more and less than 40000 mm 2 , and four or less in a case that the area of the ceramic block in the cross section is 40000 mm 2  or more and about 55000 mm 2  or less. 
     According to yet the other aspect of the present invention, a honeycomb structure includes a ceramic block. In the ceramic block, a plurality of honeycomb fired bodies are combined with one another with an adhesive layer interposed therebetween, and each of the honeycomb fired bodies has cell walls extending along a longitudinal direction of the honeycomb structure to define cells. An area of the honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in a cross section perpendicular to the longitudinal direction. An area of the ceramic block is at least about 10000 mm 2  and at most about 55000 mm 2  in the cross section. A number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from a center of gravity of the ceramic block to a periphery of the ceramic block in the cross section is two or less in a case that the area of the ceramic block in the cross section is about 10000 mm 2  or more and less than 25000 mm 2 . The number of the adhesive layers is three or less in a case that the area of the ceramic block in the cross section is 25000 mm 2  or more and less than 40000 mm 2 . The number of the adhesive layers is four or less in a case that the area of the ceramic block in the cross section is 40000 mm 2  or more and about 55000 mm 2  or less. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the first invention. 
         FIG. 2A  is a perspective view schematically showing a center-portion honeycomb fired body in the honeycomb structure according to the first embodiment of the first invention, and  FIG. 2B  is an A-A line cross-sectional view of the honeycomb fired body shown in  FIG. 2A . 
         FIG. 3  is a perspective view schematically showing a peripheral-portion honeycomb fired body according to the first embodiment of the first invention. 
         FIG. 4  is a cross-sectional view of a honeycomb structure manufactured in Example 1-1. 
         FIG. 5  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 1-1. 
         FIG. 6  is a cross-sectional view of a honeycomb structure according to the second embodiment of the first invention. 
         FIGS. 7A and 7B  are cross-sectional views for describing another example of a method for manufacturing a honeycomb structure according to the third embodiment of the first invention. 
         FIG. 8  is a cross-sectional view of a honeycomb structure according to another embodiment of the first invention. 
         FIG. 9  is a cross-sectional view of a honeycomb structure according to another embodiment of the first invention. 
         FIG. 10  is a cross-sectional view of a honeycomb structure according to another embodiment of the first invention. 
         FIG. 11  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the second invention. 
         FIG. 12  is a cross-sectional view of a honeycomb structure manufactured in Example 2-1. 
         FIG. 13  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 2-1. 
         FIG. 14  is a cross-sectional view of a honeycomb structure according to the second embodiment of the second invention. 
         FIGS. 15A and 15B  are cross-sectional views of the honeycomb structure according to another embodiment of the second invention. 
         FIGS. 16A and 16B  are cross-sectional views for describing another example of a method for manufacturing a honeycomb structure according to the embodiments of the second invention. 
         FIG. 17  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the third invention. 
         FIG. 18  is an A-A line cross-sectional view of the honeycomb structure shown in  FIG. 17 . 
         FIG. 19  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 3-1. 
         FIG. 20  is a cross-sectional view of a honeycomb structure according to the second embodiment of the third invention. 
         FIG. 21  is a cross-sectional view of a honeycomb structure according to another embodiment of the third invention. 
         FIG. 22  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the fourth invention. 
         FIG. 23  is an A-A line cross-sectional view of the honeycomb structure shown in  FIG. 22 . 
         FIG. 24  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 4-1. 
         FIG. 25  is a cross-sectional view of a honeycomb structure according to the second embodiment of the fourth invention. 
         FIG. 26  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 4-2. 
         FIG. 27  is a cross-sectional view of a honeycomb structure according to the third embodiment of the fourth invention. 
         FIGS. 28A and 28B  are cross-sectional views for describing another example of a method for manufacturing a honeycomb structure according to the third embodiment of the fourth invention. 
         FIG. 29  is a cross-sectional view of a honeycomb structure manufactured in Comparative Example 4-3. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     Upon using a honeycomb structure as an exhaust-gas purifying filter, a high-temperature exhaust gas discharged from an internal combustion engine flows into cells of the honeycomb structure. At this time, since much heat is applied to a honeycomb fired body located in the center portion, temperature of the honeycomb fired body located in the center portion tends to easily increase in comparison with that of the honeycomb fired body located in the peripheral portion. 
     Moreover, in the honeycomb structure having a plurality of honeycomb fired bodies combined with one another with an adhesive layer interposed therebetween (hereinafter, also referred to as an aggregated honeycomb structure), since normally the thermal conductivity of the adhesive layer is inferior to the thermal conductivity of the honeycomb fired bodies, the thermal conduction is easily intervened by the adhesive layer. Consequently, a great temperature difference tends to be caused between the center portion and the peripheral portion in the aggregated honeycomb structure. 
     In particular, in the honeycomb structures disclosed in WO01/23069A1, JP-A 2004-154718 and WO04/96414A1, the honeycomb fired bodies, each having a sufficiently smaller cross-sectional area than that of the honeycomb fired bodies in the center portion, are located in the peripheral portion, and since the presence of these honeycomb fired bodies having a smaller cross-sectional area located in the peripheral portion causes an increase in the ratio of occupation of the adhesive layer, the temperature difference between the center portion and the peripheral portion tends to become greater. 
     In the case when the temperature difference between the center portion and the peripheral portion of the honeycomb structure increases, upon carrying out a regenerating process on the honeycomb structure for burning and removing particulates, unburned particulates tend to remain in the peripheral portion of the honeycomb structure. 
     Upon using the honeycomb structure as an exhaust-gas purifying filter, it is required to hold the honeycomb structure in a predetermined casing with a holding sealing material. In order to prevent displacement of the honeycomb structure in the casing or to prevent coming off of a part of the honeycomb fired bodies from the honeycomb structure due to the exhaust gases, it is required to surely secure the honeycomb structure in the casing. Thus, the honeycomb structure preferably has high strength for preventing damages due to compressive stress applied from the outside of the honeycomb structure. 
     In the honeycomb structure disclosed in WO01/23069A1, JP-A 2004-154718 and WO04/96414A1, the adhesive layers are formed into a grid pattern. Thus, the honeycomb structure has high strength to compressive stress applied from a predetermined direction (a direction parallel to the adhesive layer), but has low strength to compressive stress applied from another direction, for example a direction which makes about 45° with the adhesive layer, and the honeycomb structure tends to be damaged due to the compressive stress from the direction. 
     Moreover, in the honeycomb structure disclosed in WO01/23069A1, JP-A 2004-154718 and WO04/96414A1, each of the adhesive layers crosses one another at right angles. Thus, the honeycomb structure tends to fail to spread stress generated in the honeycomb structure and the honeycomb structure tends to be damaged. 
     The inventors of the present invention have made eager investigations to solve the above problems. 
     A honeycomb structure according to an embodiment of the first aspect of the present invention includes: a plurality of honeycomb fired bodies that are combined with one another with an adhesive layer interposed therebetween, each of the honeycomb fired bodies having a large number of cells that are placed in parallel with one another in a longitudinal direction with a cell wall interposed therebetween, 
     wherein 
     the honeycomb fired bodies include a center-portion honeycomb fired body located in a center portion and a peripheral-portion honeycomb fired body located in a peripheral portion in a cross section perpendicular to the longitudinal direction of the honeycomb structure, a shape of the center-portion honeycomb fired body is a substantially rectangular shape in the cross section, 
     an area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in the cross section, 
     a shape of the peripheral-portion honeycomb fired body is different from the shape of the center-portion honeycomb fired body in the cross section, and 
     an area of the peripheral-portion honeycomb fired body is at least about 0.9 times and at most about 1.3 times larger than the area of the center-portion honeycomb fired body in the cross section. 
     In the honeycomb structure according to the embodiment of the first aspect of the present invention, out of the plurality of the honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween, the peripheral-portion honeycomb fired body has the area at least about 0.9 times and at most about 1.3 times larger than the area of the center-portion honeycomb fired body in the cross section. Therefore, since no honeycomb fired body having an extremely small cross-sectional area is located in the peripheral portion of the honeycomb structure and since the adhesive layer to be used for combining such small honeycomb fired bodies with one another is not required, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     Further, since the area of the peripheral-portion honeycomb fired body is about 0.9 times or more larger than the area of the center-portion honeycomb fired body in the cross section, a temperature distribution tends not to occur between the honeycomb fired body located in the center portion and that located in the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     Moreover, since the area of the peripheral-portion honeycomb fired body is up to about 1.3 times larger than the area of the center-portion honeycomb fired body in the cross section, cracks tend not to occur in the honeycomb fired body due to thermal stress. 
     Furthermore, in the honeycomb structure according to the embodiment of the first aspect of the present invention, the area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in the cross section. The reason for this structure is described as follows. 
     In the case that the cross-sectional area of the center-portion honeycomb fired body is about 900 mm 2  or more, an amount of adhesive for forming the honeycomb structure tends not to become large, with the result that a temperature distribution tends not to occur in the honeycomb structure and cracks tend not to occur in the honeycomb fired body upon carrying out a regenerating process. 
     In contrast, in the case that the cross-sectional area of the center-portion honeycomb fired body is about 2500 mm 2  or less, the effect of the adhesive layer for alleviating the thermal stress is sufficient and cracks tend not to occur in the honeycomb fired body. That is, the cross-sectional area of the center-portion honeycomb fired body maintained within the above range is suitable for preventing the occurrence of cracks in the honeycomb fired body upon carrying out the regenerating process. 
     In the honeycomb structure according to the embodiment of the first aspect of the present invention, the shape of the peripheral-portion honeycomb fired body is preferably formed into a shape surrounded by three line segments and one arc or elliptical arc in the cross section, and 
     two angles made by the two line segments out of the three line segments are a substantially right angle and an obtuse angle. 
     In the case that the peripheral-portion honeycomb fired body has the shape of this kind, the size of the peripheral-portion honeycomb fired body in the cross section tends not to be extremely small in comparison with that of the center-portion honeycomb fired body. Therefore, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     A honeycomb structure according to an embodiment of the second aspect of the present invention having a substantially round pillar-shape or substantially cylindroid shape includes: 
     a ceramic block in which
         a plurality of honeycomb fired bodies are combined with one another with an adhesive layer interposed therebetween, and   each of the honeycomb fired bodies has a large number of cells that are placed in parallel with one another in a longitudinal direction with a cell wall interposed therebetween,       

     wherein 
     a plurality of the honeycomb fired bodies include a center-portion honeycomb fired body located in a center portion of the ceramic block and a peripheral-portion honeycomb fired body forming a part of a peripheral side face of the ceramic block, 
     an area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in a cross section perpendicular to the longitudinal direction, and 
     provided that a figure, which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, a part of the peripheral-portion honeycomb fired body is located in the figure. 
     In the honeycomb structure according to the embodiment of the second aspect of the present invention, the plurality of the honeycomb fired bodies are combined with one another with the adhesive layer interposed therebetween, and the plurality of the honeycomb fired bodies include the center-portion honeycomb fired body and the peripheral-portion honeycomb fired body. 
     In the honeycomb structure, provided that a figure, which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, a part of the peripheral-portion honeycomb fired body is located in the figure. 
     In the structure of this kind, in the cross section perpendicular to the longitudinal direction of the honeycomb structure including the center-portion honeycomb fired body and the peripheral-portion honeycomb fired body, since there is no peripheral-portion honeycomb fired body which is located only outside the figure, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain. 
     As mentioned above, temperature of the center-portion honeycomb fired body tends to increase more easily than that of the peripheral-portion honeycomb fired body in the honeycomb structure. 
     When a part of each of the peripheral-portion honeycomb fired bodies is located in the figure, heat tends to be transferred to the peripheral-portion honeycomb fired bodies, and thus, unburned particulates tend not to remain. 
     On the other hand, when a part of each of the peripheral-portion honeycomb fired bodies is not located in the figure (each of the honeycomb fired bodies is located only outside the figure), the honeycomb structure tends to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tends to remain as mentioned above. 
     Furthermore, in the honeycomb structure according to the embodiment of the second aspect of the present invention, the area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in the cross section. The reason for this structure is described as follows. 
     In the case that the cross-sectional area of the center-portion honeycomb fired body is about 900 mm 2  or more, an amount of adhesive for forming the honeycomb structure tends not to become large, with the result that a temperature distribution tends not to occur in the honeycomb structure and cracks tend not to occur in the honeycomb fired body upon carrying out a regenerating process. 
     In contrast, in the case that the cross-sectional area of the center-portion honeycomb fired body is about 2500 mm 2  or less, the effect of the adhesive layer for alleviating the thermal stress is sufficient and cracks tend not to occur in the honeycomb fired body. That is, the cross-sectional area of the center-portion honeycomb fired body maintained within the above range is suitable for preventing the occurrence of cracks in the honeycomb fired body upon carrying out the regenerating process. 
     A honeycomb structure according to an embodiment of the second aspect of the present invention preferably has a substantially round pillar-shape or a substantially cylindroid shape. 
     A honeycomb structure according to an embodiment of the third aspect of the present invention includes: a plurality of honeycomb fired bodies that are combined with one another with an adhesive layer interposed therebetween, each of the honeycomb fired bodies having a large number of cells that are placed in parallel with one another in a longitudinal direction with a cell wall interposed therebetween, 
     wherein 
     the honeycomb structure includes: a peripheral portion forming a peripheral side face of the honeycomb structure; and a center portion having a substantially rectangular shape located at the inner side of the peripheral portion in a cross section perpendicular to the longitudinal direction of the honeycomb structure, 
     the peripheral portion includes a plurality of peripheral-portion honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween, 
     the center portion includes one center-portion honeycomb fired body or a plurality of center-portion honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween, 
     the honeycomb structure includes at least one of the adhesive layers in the peripheral portion formed in a direction extending from a corner point of the center portion to the peripheral side face of the honeycomb structure in the cross section, and 
     the adhesive layer extending from the corner point of the center portion to the peripheral side face of the honeycomb structure forms an angle of at least about 40° and at most about 50° with at least one adhesive layer formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure. 
     With respect to the honeycomb structure according to the embodiment of the third aspect of the present invention, of the adhesive layers in the peripheral portion, the adhesive layer formed in a direction extending from a corner point of the center portion to the peripheral side face of the honeycomb structure is also referred to as a “first peripheral-portion adhesive layer”, and the adhesive layer formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure is also referred to as a “second peripheral-portion adhesive layer”, hereinafter. 
     Also with respect to the honeycomb structure according to the embodiment of the third aspect of the present invention, the center portion in the cross section perpendicular to the longitudinal direction of the honeycomb structure is the area occupied by: the center-portion honeycomb fired body; the adhesive layer combining the center-portion honeycomb fired bodies with one another; and the adhesive layer combining the center-portion honeycomb fired body with the peripheral-portion honeycomb fired body. 
     Furthermore, the peripheral portion in the cross section perpendicular to the longitudinal direction of the honeycomb structure is the area occupied by: the peripheral-portion honeycomb fired bodies; and the adhesive layer combining the peripheral-portion honeycomb fired bodies with one another. 
     The honeycomb structure according to the embodiment of the third aspect of the present invention has the center portion and the peripheral portion, and in the peripheral portion located outside the center portion, the plurality of the peripheral-portion honeycomb fired bodies forming a part of the peripheral side face of the honeycomb structure are combined with one another with the adhesive layer interposed therebetween. 
     Of the adhesive layers interposed between the peripheral-portion honeycomb fired bodies in the cross section perpendicular to the longitudinal direction of the honeycomb structure, the angle formed by the adhesive layer extending from the corner point of the center portion to the peripheral side face of the honeycomb structure (the first peripheral-portion adhesive layer) and at least one adhesive layer formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure (the second peripheral-portion adhesive layer) is at least about 40° and at most about 50°. 
     Thus, it is easier to prevent the honeycomb structure from being damaged due to compressive stress applied from the outside of the honeycomb structure. 
     Further, since the first peripheral-portion adhesive layer extends from the corner point of the center portion to the peripheral side face of the honeycomb structure, two adhesive layers existing between the center-portion honeycomb fired body and the peripheral-portion honeycomb fired body and the first peripheral-portion adhesive layer form a Y shape in the corner point of the center portion. 
     As mentioned above, in the case that there is the Y-shape portion of the adhesive layer in the cross section perpendicular to the longitudinal direction of the honeycomb structure, it is easier to prevent the honeycomb structure from being damaged. 
     In the honeycomb structure according to the embodiment of the third aspect of the present invention, the angle formed by the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer means the angle formed by the straight line passing through the inside of the first peripheral-portion adhesive layer and the straight line passing through the inside of the second peripheral-portion adhesive layer. 
     In the honeycomb structure according to the embodiment of the third aspect of the present invention, preferably, the center portion includes a plurality of the center-portion honeycomb fired bodies combined with one another with the adhesive layer interposed therebetween, and 
     in the cross section perpendicular to the longitudinal direction of the honeycomb structure, at least one adhesive layer, which is disposed between the peripheral-portion honeycomb fired bodies and formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure, forms a substantially straight line with at least one adhesive layer disposed between the center-portion honeycomb fired bodies. 
     The adhesive layer of this kind is more likely to play a role as, so as to say, a beam for improving strength of the honeycomb structure. 
     A honeycomb structure according to an embodiment of the fourth aspect of the present invention includes: 
     a ceramic block in which
         a plurality of honeycomb fired bodies are combined with one another with an adhesive layer interposed therebetween, and   each of the honeycomb fired bodies has a large number of cells that are placed in parallel with one another in a longitudinal direction with a cell wall interposed therebetween,       

     wherein 
     an area of the honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2  in a cross section perpendicular to the longitudinal direction, 
     an area of the ceramic block is at least about 10000 mm 2  and at most about 55000 mm 2  in the cross section, and 
     the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section is:
         two or less in the case that the area of the ceramic block in the cross section is about 10000 mm 2  or more and less than 25000 mm 2 ,   three or less in the case that the area of the ceramic block in the cross section is 25000 mm 2  or more and less than 40000 mm 2 , and   four or less in the case that the area of the ceramic block in the cross section is 40000 mm 2  or more and about 55000 mm 2  or less.       

     With respect to the honeycomb structure according to the embodiment of the fourth aspect of the present invention, in the case that the center of gravity is on the adhesive layer upon counting the number of adhesive layers which exist on the route extending from the center of gravity of the ceramic block to the periphery of the ceramic block, the adhesive layer on which the center of gravity exists is counted as one of adhesive layers existing on the route. 
     Also with respect to the honeycomb structure according to the embodiment of the fourth aspect of the present invention, upon counting the number of adhesive layers which exist on the route extending from the center of gravity of the ceramic block to the periphery of the ceramic block, the route is decided so as to pass through the smallest number of the adhesive layers. 
     The honeycomb structure according to the embodiment of the fourth aspect of the present invention includes the ceramic block in which the plurality of the honeycomb fired bodies are combined with one another with the adhesive layer interposed therebetween, and in the honeycomb structure, the area of the honeycomb fired body is about 900 mm 2  and at most about 2500 mm 2  in the cross section, and the area of the ceramic block is at least about 10000 mm 2  and at most about 55000 mm 2  in the cross section. 
     In the honeycomb structure of this kind, since the cross-sectional area of the ceramic block and the number of the adhesive layers which exist on the route extending from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section perpendicular to the longitudinal direction of the honeycomb structure satisfy the above-mentioned relationships, the honeycomb structure is allowed to exert the following effects: 
     the adhesive layer easily alleviates thermal stress, and thus, it is possible to prevent occurrence of cracks and damages on the honeycomb structure; and 
     the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion of the honeycomb structure, and thus, unburned particulates tend not to remain. 
     That is, in the honeycomb structure according to the embodiment of the fourth aspect of the present invention, since the route extending from the center portion to the peripheral portion of the honeycomb structure (main route of heat transfer) is decided so as to pass through the adhesive layers as small in number as possible, and the honeycomb structure tends not to impair a function to alleviate thermal stress of the adhesive layer, heat is easily transferred from the center portion to the peripheral portion of the honeycomb structure, and thus, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion. Moreover, it is easier to prevent occurrence of damages and cracks in the honeycomb structure. 
     In the honeycomb structure according to the embodiment of the fourth aspect of the present invention, the ceramic block preferably has a substantially round shape in the cross section. 
     The following effect is allowed to be exerted particularly in the case that the ceramic block has a substantially round shape in the cross section, that is, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion when the cross-sectional area of the ceramic block and the number of the adhesive layers which exist on the route extending from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section of the honeycomb structure satisfy the above-mentioned relationships. 
     This is because, although the peripheral portion of the honeycomb block tends to include a honeycomb fired body having a small cross-sectional area in the case that the honeycomb block has a substantially round cross-sectional shape, the honeycomb structure satisfying the above relationships easily avoids the tendency of this kind. 
     In the present description, the cross section perpendicular to the longitudinal direction of the honeycomb structure, the cross section perpendicular to the longitudinal direction of the ceramic block, the cross section perpendicular to the longitudinal direction of the honeycomb fired body, and the cross section perpendicular to the longitudinal direction of the honeycomb molded body may be simply referred to as the cross section of a honeycomb structure, the cross section of a ceramic block, the cross section of a honeycomb fired body, and the cross section of a honeycomb molded body. 
     Moreover, in the present description, the cross-sectional area of a honeycomb structure, the cross-sectional area of a ceramic block, the cross-sectional area of a honeycomb fired body, and the cross-sectional area of a honeycomb molded body may be simply referred to as the cross-sectional area perpendicular to the longitudinal direction of the honeycomb structure, the cross-sectional area perpendicular to the longitudinal direction of the ceramic block, the cross-sectional area perpendicular to the longitudinal direction of the honeycomb fired body, and the cross-sectional area perpendicular to the longitudinal direction of the honeycomb molded body. 
     In the present description, the center-portion honeycomb fired body refers to a honeycomb fired body that does not form the periphery of the honeycomb structure in the cross section perpendicular to the longitudinal direction of the honeycomb structure, and the peripheral-portion honeycomb fired body refers to a honeycomb fired body that forms the periphery of the honeycomb structure in the cross section perpendicular to the longitudinal direction of the honeycomb structure. 
     Here, in the case that a coat layer is formed on the honeycomb structure as will be described later, the peripheral-portion honeycomb fired body refers to a honeycomb fired body that forms the periphery of a ceramic block. 
     As mentioned above, the honeycomb fired bodies used for forming the honeycomb structure according to each of the embodiments of the first to third aspects of the present invention are distinguished as the center-portion honeycomb fired bodies and the peripheral-portion honeycomb fired bodies. 
     However, in the present description, when the two kinds of honeycomb fired bodies are not particularly required to be distinguished, each of these is simply referred to as the honeycomb fired body. 
     Referring to the drawings, the following description will discuss an embodiment of a honeycomb structure according to the first aspect of the present invention. 
     First Embodiment of First Aspect of the Present Invention 
       FIG. 1  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the first aspect of the present invention. 
       FIG. 2A  is a perspective view schematically showing a center-portion honeycomb fired body in the honeycomb structure according to the first embodiment of the first aspect of the present invention and  FIG. 2B  is an A-A line cross-sectional view of the honeycomb fired body shown in  FIG. 2A . 
       FIG. 3  is a perspective view schematically showing a peripheral-portion honeycomb fired body according to the first embodiment of the first aspect of the present invention. 
     In a honeycomb structure  100  shown in  FIG. 1 , a plurality of center-portion honeycomb fired bodies  110  having a shape shown in  FIGS. 2A and 2B  and a plurality of peripheral-portion honeycomb fired bodies  120  having a shape shown in  FIG. 3  are combined with one another, with an adhesive layer  101  interposed therebetween, to form a ceramic block  103 . A coat layer  102  is further formed on the periphery of the ceramic block  103 . 
     The shape of the cross section of each of the center-portion honeycomb fired bodies  110  is a substantially square shape. 
     The shape of the cross section of each of the peripheral-portion honeycomb fired bodies  120  is formed into a shape surrounded by three line segments  120   a ,  120   b  and  120   c  and an arc  120   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  120   b  and  120   c  and an angle made by the line segments  120   a  and  120   b ) are about 90° and about 135°. 
     The honeycomb fired bodies  110  and  120  include porous silicon carbide sintered bodies. 
     The center-portion honeycomb fired body  110  shown in  FIGS. 2A and 2B  has a structure in which a large number of cells  111  are longitudinally placed (the direction indicated by an arrow a in  FIG. 2A ) in parallel with one another with a cell wall  113  therebetween, the cells  111  having either one of the ends sealed with a plug  112 . Therefore, exhaust gas G having flown into one cell  111  with an opening on one end face (see an arrow in  FIG. 2B ) flow out from another cell  111  with an opening on the other end face after having always passed through the cell wall  113  that separates the cells  111 . 
     Therefore, the cell wall  113  functions as a filter for capturing PM and the like. 
     In the same manner as in the center-portion honeycomb fired body  110 , the peripheral-portion honeycomb fired body  120  shown in  FIG. 3  has a structure in which a large number of cells  121  are longitudinally placed in parallel with one another with a cell wall  123  therebetween, and the cells  121  having either one of the ends sealed with a plug  122 . Therefore, exhaust gas having flown into one cell  121  with an opening on one end face flows out from another cell  121  with an opening on the other end face after having always passed through a cell wall  123  that separates the cells  121 . 
     That is, although the outer shape of the peripheral-portion honeycomb fired body  120  is different from that of the center-portion honeycomb fired body  110 , the peripheral-portion honeycomb fired body  120  has the same functions as those of the center-portion honeycomb fired body  110 . 
     As shown in  FIG. 1 , in the honeycomb structure  100 , four pieces of the center-portion honeycomb fired bodies  110  are located in the center portion of the cross section of the honeycomb structure  100 , and eight pieces of the peripheral-portion honeycomb fired bodies  120  are located on the periphery of the four pieces of the center-portion honeycomb fired bodies  110 . These honeycomb fired bodies are combined with one another with the adhesive layer  101  interposed therebetween so that the cross section of the honeycomb structure  100  (ceramic block  103 ) is formed into a substantially round shape. 
     In the honeycomb structure  100 , the shape of the cross section of the peripheral-portion honeycomb fired body  120  is different from that of the center-portion honeycomb fired body  110 , and the cross-sectional area of the peripheral-portion honeycomb fired body  120  is at least about 0.9 times and at most about 1.3 times larger than that of the center-portion honeycomb fired body  110 . 
     Therefore, no honeycomb fired bodies having an extremely small cross-sectional area are located in the peripheral portion of the honeycomb structure  100 , and of course, an adhesive layer to be used for combining such small honeycomb fired bodies with one another is not required. For this reason, the honeycomb structure  100  tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     As mentioned above, the cross section of the peripheral-portion honeycomb fired body  120  is formed into the shape surrounded by the three line segments  120   a ,  120   b  and  120   c  and an arc  120   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  120   b  and  120   c  and an angle made by the line segments  120   a  and  120   b ) are about 90° and about 135°. The fact that the shape of the peripheral-portion honeycomb fired body  120  is formed into this shape is also one reason why no honeycomb fired body having an extremely small cross-sectional area is located in the peripheral portion of the honeycomb structure  100 . 
     Moreover, in the honeycomb structure  100 , the cross-sectional area of the center-portion honeycomb fired body  110  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     By setting the cross-sectional area of the center-portion honeycomb fired body  110  to such a size, it becomes easier to prevent cracks from occurring in the honeycomb structure  100  upon carrying out a regenerating process on the honeycomb structure  100 . 
     The following description will discuss a method for manufacturing a honeycomb structure of the present embodiment. 
     (1) A molding process is carried out in which a wet mixture containing ceramic powders and a binder is extrusion-molded to manufacture a honeycomb molded body. 
     More specifically, first, as ceramic powders, silicon carbide powders each having a different average particle diameter, an organic binder, a liquid-state plasticizer, a lubricant and water are mixed to prepare a wet mixture used for manufacturing a honeycomb molded body. 
     Successively, this wet mixture is charged into an extrusion molding apparatus. When the wet mixture is charged into the extrusion molding apparatus, the wet mixture is extrusion-molded into a honeycomb molded body having a predetermined shape. 
     In order to manufacture a honeycomb molded body having a variety of cross-sectional shapes, extrusion-molding dies corresponding to the respective shapes are used. The variety of cross-sectional shapes include a square cross-sectional shape and a shape surrounded by three line segments and an arc, with the two angles (made by two line segments out of these three line segments) being about 90° and about 135°. 
     (2) Next, the honeycomb molded body thus formed is cut into a predetermined length, and undergoes a drying process by using a drying apparatus, such as a microwave drying apparatus, a hot-air drying apparatus, a dielectric drying apparatus, a reduced-pressure drying apparatus, a vacuum drying apparatus and a freeze drying apparatus. Then, this dried honeycomb molded body undergoes a sealing process in which predetermined cells are filled with a plug material paste to be formed into plugs to seal the cells. 
     Here, with respect to the conditions of the cutting process, drying process and sealing process, those conditions conventionally used upon manufacturing a honeycomb fired body can be adopted. 
     (3) Next, the honeycomb molded body undergoes a degreasing process in which the organic substances therein are heated in a degreasing furnace, and is then transported to a firing furnace, and undergoes a firing process therein to manufacture a honeycomb fired body. 
     Here, with respect to the conditions of the degreasing process and firing process, those conditions conventionally used upon manufacturing a honeycomb fired body can be adopted. 
     By carrying out the above-mentioned processes, the center-portion honeycomb fired body and the peripheral-portion honeycomb fired body are manufactured. 
     (4) Next, an adhesive paste was applied to a predetermined side surface of each of the center-portion honeycomb fired body and each of the peripheral-portion honeycomb fired body, with the predetermined end portion of each of the cells sealed, to form an adhesive paste layer. After this, another honeycomb fired body is piled up onto the above-mentioned adhesive paste layer sequentially. By carrying out the above process repeatedly, the combining process is carried out to manufacture a ceramic block in which a predetermined number of the honeycomb fired bodies are combined with one another. 
     With respect to the adhesive paste, the adhesive paste including an inorganic binder, an organic binder, and inorganic particles may be used, for example. Moreover, the adhesive paste may further include at least one of inorganic fibers and whiskers. 
     (5) Subsequently, a coat layer forming process is further carried out in which a coating material paste is applied to the periphery of the ceramic block formed into the substantially round pillar shape, and is dried and solidified to form a coat layer. 
     Here, the same paste as the adhesive paste may be used as the coating material paste. Alternatively, a paste having a different composition from the composition of the adhesive paste may be used as the coating material paste. 
     It is not necessarily required to form the coat layer, and the coat layer may be formed, on demand. 
     It is possible to manufacture the honeycomb structure of the present embodiment through the above-mentioned processes. 
     The following description will summarize the effects of the honeycomb structure of the present embodiment. 
     (1) In the honeycomb structure of the present embodiment, the cross-sectional shape of the peripheral-portion honeycomb fired body  120  is different from the cross-sectional shape of the center-portion honeycomb fired body  110 , and the cross-sectional area of the peripheral-portion honeycomb fired body  120  is at least about 0.9 times and at most about 1.3 times larger than the cross-sectional area of the center-portion honeycomb fired body. Therefore, since no honeycomb fired body having an extremely small cross-sectional area is located in the peripheral portion of the honeycomb structure and since the adhesive layer to be used for combining such small honeycomb fired bodies with one another is not required, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     (2) In the honeycomb structure of the present embodiment, the cross-sectional shape of the peripheral-portion honeycomb fired body is formed into a shape that is surrounded by three line segments and an arc. The two angles made by two line segments out of these three line segments are about 900 and about 135°. For this reason, it is possible to avoid the cross-sectional area of the peripheral-portion honeycomb fired body from becoming extremely small in comparison with the cross-sectional area of the center-portion honeycomb fired body. Moreover, the adhesive layer used for combining the honeycomb fired bodies having a small cross-sectional area with one another is not required. Therefore, the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     (3) In the honeycomb structure of the present embodiment, the cross-sectional area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2 . For this reason, cracks tend not to occur in the honeycomb fired body upon carrying out a regenerating process. 
     (4) In the honeycomb fired body of the honeycomb structure of the present embodiment, either one end of each of the cells is sealed with a plug. Therefore, the honeycomb structure of the present embodiment is more likely to be suitably used as a diesel particulate filter. 
     (5) In the honeycomb structure of the present embodiment, since the coat layer is formed on the peripheral side face of the ceramic block, it is easier to prevent leakage of particulates from the peripheral side face of the honeycomb structure. 
     Example 1-1 
     The following description will discuss examples that specifically disclose the first embodiment of the first aspect of the present invention. Here, the first aspect of the present invention is not limited to these examples. 
     (1) An amount of 52.8% by weight of a silicon carbide coarse powder having an average particle diameter of 22 μm and 22.6% by weight of a silicon carbide fine powder having an average particle diameter of 0.5 μm were mixed. To the resulting mixture, 2.1% by weight of an acrylic resin, 4.6% by weight of an organic binder (methylcellulose), 2.8% by weight of a lubricant (UNILUB, manufactured by NOF Corporation), 1.3% by weight of glycerin, and 13.8% by weight of water were added, and then kneaded to prepare a wet mixture. The obtained wet mixture was extrusion-molded. 
     In this process, there have been manufactured: a raw honeycomb molded body having approximately the same shape as that of the center-portion honeycomb fired body  110  illustrated in  FIGS. 2A and 2B  with cells not sealed; and a raw honeycomb molded body having approximately the same shape as that of the peripheral-portion honeycomb fired body  120  illustrated in  FIG. 3  with cells not sealed. 
     (2) Next, the raw honeycomb molded bodies were dried by using a microwave drying apparatus to obtain dried honeycomb molded bodies. A paste having the same composition as that of the wet mixture was then filled into predetermined cells, and the filled portions of the dried honeycomb molded bodies were dried by using a drying apparatus again. 
     (3) The dried honeycomb molded bodies were degreased at 400° C., and then fired at 2200° C. under normal pressure argon atmosphere for three hours. 
     Thus, a center-portion honeycomb fired body  110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.3 mm×34.3 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  120   a= 20.8 mm, line segment  120   b =35.0 mm, and line segment  120   c =35.7 mm) was manufactured. 
     Here, the cross-sectional area of the center-portion honeycomb fired body  110  was 1190 mm 2  and the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1292 mm 2 . Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1.09 times larger than the cross-sectional area of the center-portion honeycomb fired body  110 . 
     (4) An adhesive paste was applied to predetermined side faces of the center-portion honeycomb fired body  110  and the peripheral-portion honeycomb fired body  120 , and four pieces of the center-portion honeycomb fired bodies  110  and eight pieces of the peripheral-portion honeycomb fired bodies  120  were bonded to one another with the adhesive paste interposed therebetween so as to be arranged as shown in  FIG. 4 . The adhesive paste was solidified at 180° C. in 20 minutes to manufacture a round pillar-shaped ceramic block  103  having the adhesive layer 1 mm in thickness. 
     Here, as the adhesive paste, an adhesive paste containing 30.0% by weight of silicon carbide particles having an average particle diameter of 0.6 μm, 21.4% by weight of silica sol, 8.0% by weight of carboxy methylcellulose and 40.6% by weight of water, was used. 
     (5) By using a coating material paste having the same composition as that of the adhesive paste used in the process (4), a coating material paste layer was formed on the periphery of the ceramic block  103 . Thereafter, the coating material paste layer was dried at 120° C. to manufacture a round pillar-shaped honeycomb structure having a size of 143.8 mm in diameter×150 mm in length with a coat layer  102  formed on the periphery thereof. 
     The honeycomb structure  100  manufactured in Example 1 has a cross-sectional shape as shown in  FIG. 4 . 
     Example 1-2 
     A honeycomb structure was manufactured in the same manner as in Example 1-1, except that the sizes of a center-portion honeycomb fired body  110  and a peripheral-portion honeycomb fired body  120 , each manufactured through the processes (1) to (3) of Example 1-1, were changed to the below-mentioned sizes. 
     A center-portion honeycomb fired body  110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 36.7 mm×36.7 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  120   a =17.7 mm, line segment  120   b =37.2 mm and line segment  120   c =33.5 mm) was manufactured. 
     Here, the cross-sectional area of the center-portion honeycomb fired body  110  was 1347 mm 2 , and the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1215 mm 2 . Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 0.90 times larger than the cross-sectional area of the center-portion honeycomb fired body  110 . 
     Example 1-3 
     A honeycomb structure was manufactured in the same manner as in Example 1-1, except that the sizes of a center-portion honeycomb fired body  110  and a peripheral-portion honeycomb fired body  120 , each manufactured through the processes (1) to (3) of Example 1-1, were changed to the below-mentioned sizes. 
     A center-portion honeycomb fired body  110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 32.4 mm×32.4 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  120   a =23.8 mm, line segment  120   b =32.9 mm and line segment  120   c =37.8 mm) was manufactured. 
     Here, the cross-sectional area of the center-portion honeycomb fired body  110  was 1050 mm 2 , and the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1363 mm 2 . Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1.30 times larger than the cross-sectional area of the center-portion honeycomb fired body  110 . 
     Comparative Example 1-1 
     (1) By carrying out the same processes as the processes (1) to (3) of Example 1-1, a honeycomb fired body including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     (2) An adhesive paste was applied to a predetermined side face of the honeycomb fired body, and 16 pieces of honeycomb fired bodies were bonded to one another with the adhesive paste interposed therebetween. The adhesive paste was solidified at 180° C. in 20 minutes to manufacture an aggregated body of the honeycomb fired bodies having a rectangular pillar-shape, with the thickness of the adhesive layer being 1 mm. 
     Here, as the adhesive paste, the same adhesive paste as that used in Example 1-1 was used. 
     (3) Next, the periphery of the aggregated body of the honeycomb fired bodies was cut by using a diamond cutter, to manufacture a round pillar-shaped ceramic block. 
     Subsequently, a coating material paste layer was formed on the periphery of the ceramic block by using the coating material paste made of the same material as that of the adhesive paste. Further, this coating material paste layer was dried at a temperature of 120° C. to manufacture a round pillar-shaped honeycomb structure having a size of 143.8 mm in diameter×150 mm in length, with a coat layer formed on the periphery thereof. 
     The cross-sectional shape of the honeycomb structure manufactured in Comparative Example 1-1 is shown in  FIG. 5 . 
       FIG. 5  is a cross-sectional view that shows the honeycomb structure  400  manufactured in Comparative Example 1-1, and in  FIG. 5 , a reference numeral  410  represents a center-portion honeycomb fired body, reference numerals  420  and  430  represent peripheral-portion honeycomb fired bodies, a reference numeral  401  represents an adhesive layer, a reference numeral  402  represents a coat layer and a reference numeral  403  represents a ceramic block. 
     In the honeycomb structure  400 , the cross-sectional area of the center-portion honeycomb fired body  410  is 1190.5 mm 2 , the cross-sectional area of the peripheral-portion honeycomb fired body  420  is 1095 mm 2 , and the cross-sectional area of the peripheral-portion honeycomb fired body  430  is 357 mm 2 . 
     Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  420  is 0.92 times larger than the cross-sectional area of the center-portion honeycomb fired body  410 , and the cross-sectional area of the peripheral-portion honeycomb fired body  430  is 0.30 times larger than the cross-sectional area of the center-portion honeycomb fired body  410 . 
     Comparative Example 1-2 
     A honeycomb structure was manufactured in the same manner as in Example 1-1, except that the sizes of a center-portion honeycomb fired body  110  and a peripheral-portion honeycomb fired body  120 , each manufactured through the processes (1) to (3) of Example 1-1, were changed to the below-mentioned sizes. 
     A center-portion honeycomb fired body  110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 31.5 mm×31.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  120   a =25.0 mm, line segment  120   b =32.0 mm and line segment  120   c =38.2 mm) was manufactured. 
     Here, the cross-sectional area of the center-portion honeycomb fired body  110  was 992 mm 2 , and the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1392 mm 2 . Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  120  was 1.40 times larger than the cross-sectional area of the center-portion honeycomb fired body  110 . 
     (Evaluation of Honeycomb Structure) 
     A regenerating process was carried out on each of the honeycomb structures manufactured in Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 by the following method, and a regenerating rate (%) was measured by the following method based on weight differences before and after the regenerating process. 
     Here, the smaller the regenerating rate is, the more the particulates remain. 
     (Regenerating Process) 
     Each of the honeycomb structures according to Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 was placed in an exhaust passage of a 2 L engine, and a commercially available catalyst supporting carrier including a honeycomb structure made of cordierite (diameter: 200 mm, length: 100 mm, cell density: 400 pcs/inch 2 , amount of supported platinum: 5 g/L) was placed in the exhaust passage of the engine at a position closer to a gas-inlet side than the previously-placed honeycomb structure as an exhaust gas purifying apparatus. Particulates were captured for 7 hours, while the engine was driven at the number of revolutions of 3000 min −1  with a torque of 50 Nm. The amount of the captured particulates was 8 g/L. 
     Thereafter, the engine was driven at the number of revolutions of 1250 min −1  with a torque of 60 Nm, and when the temperature of the filter became constant, the state was kept for one minute. Thereafter, a post injection was performed, and then the temperature of exhaust gas was raised by utilizing the oxidation catalyst present at the front side of the exhaust gas purifying apparatus to burn particulates. 
     The conditions for the post injection were set so that the temperature of the exhaust gases flowing in the honeycomb structure became almost constant at 600° C. after a lapse of one minute from the start. 
     (Calculation of Regenerating Rate) 
     Provided that the initial weight of a honeycomb structure prior to capturing particulates is W 0 , that the weight of the honeycomb structure prior to a regenerating process after capturing particulates is W 1 , and that the weight of the honeycomb structure after the regenerating process is W 2 , the regenerating rate was calculated by using the following equation (1): 
       Regenerating rate=[( W   1   −W   0 )−( W   2   −W   0 )]/( W   1   −W   0 )  (1). 
     As a result, the regenerating rate of the honeycomb structure of Example 1-1 was 85%. 
     The regenerating rate of the honeycomb structure of Example 1-2 was 80%. 
     The regenerating rate of the honeycomb structure of Example 1-3 was 88%. 
     In contrast, the regenerating rate of the honeycomb structure of Comparative Example 1-1 was 70%. 
     Moreover, although the regenerating rate of the honeycomb structure of Comparative Example 1-2 was 90%, cracks occurred in a part of the peripheral-portion honeycomb fired body after the regenerating process. 
     Here, in the honeycomb structures of Examples 1-1 to 1-3 and Comparative Example 1-1, no cracks occurred in the honeycomb fired bodies after the regenerating process. 
     The reason that the regenerating rate was low in the honeycomb structure in Comparative Example 1-1 is presumably because a large amount of unburned particulates remained upon carrying out the regenerating process. Moreover, the reason that cracks were observed in the honeycomb structure of Comparative Example 1-2 is presumably because the cross-sectional area of the peripheral-portion honeycomb fired body was too large relative to the cross-sectional area of the center-portion honeycomb fired body. 
     Second Embodiment of First Aspect of the Present Invention 
     Referring to the drawings, the following description will discuss a second embodiment that is another embodiment of the honeycomb structure of the first aspect of the present invention. 
       FIG. 6  is a cross-sectional view of a honeycomb structure according to the second embodiment of the first aspect of the present invention. 
     As shown in  FIG. 6 , the honeycomb structure  200  of the present embodiment has a structure in which a plurality of center-portion honeycomb fired bodies  210  and pluralities of peripheral-portion honeycomb fired bodies  220  and  230  are combined with one another with an adhesive layer  201  interposed therebetween to form a ceramic block  203 . A coat layer  202  is formed on the periphery of the ceramic block  203 . 
     The cross section of each of the center-portion honeycomb fired bodies  210  has a substantially square shape. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  220  is formed into a shape surrounded by three line segments  220   a ,  220   b  and  220   c  and an arc  220   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  220   a  and  220   b  and an angle made by the line segments  220   b  and  220   c ) are about 90°. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  230  is formed into a shape surrounded by three line segments  230   a ,  230   b  and  230   c  and an arc  230   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  230   b  and  230   c  and an angle made by the line segments  230   a  and  230   b ) are about 90° and about 135°. 
     The center-portion honeycomb fired body  210  is the same as the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment. The peripheral-portion honeycomb fired bodies  220  and  230  have the same functions as that of the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment although outer shapes of those peripheral-portion honeycomb fired bodies are different from that of the center-portion honeycomb fired body  110 . 
     Moreover, the honeycomb fired bodies  210 ,  220  and  230  include porous silicon carbide sintered bodies. 
     As shown in  FIG. 6 , in the honeycomb structure  200 , nine pieces of the center-portion honeycomb fired bodies  210  are located in the center portion of the cross section of the honeycomb structure  200 , and eight pieces of the peripheral-portion honeycomb fired bodies  220  and eight pieces of the peripheral-portion honeycomb fired bodies  230  are located on the periphery of the nine pieces of center-portion honeycomb fired bodies  210 . These honeycomb fired bodies are combined with one another with the adhesive layer  201  interposed therebetween so that the cross section of the honeycomb structure  200  (ceramic block  203 ) is formed into a substantially round shape. 
     In the honeycomb structure  200 , the cross-sectional shape of each of the peripheral-portion honeycomb fired bodies  220  and  230  is different from that of the center-portion honeycomb fired body  210 . The cross-sectional area of each of the peripheral-portion honeycomb fired bodies  220  and  230  is at least about 0.9 times and at most about 1.3 times larger than that of the center-portion honeycomb fired body  210 . 
     Therefore, no honeycomb fired body having an extremely small cross-sectional area is located in the peripheral portion of the honeycomb structure  200 , and of course, an adhesive layer to be used for combining such small honeycomb fired bodies with one another is not required. For this reason, the honeycomb structure  200  tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out the regenerating process. 
     As mentioned above, the cross section of the peripheral portion honeycomb fired body  220  is formed into the shape surrounded by the three line segments  220   a ,  220   b  and  220   c  and an arc  220   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  220   a  and  220   b  and an angle made by the line segments  220   b  and  220   c ) are about 90°. As mentioned above, the cross section of the peripheral portion honeycomb fired body  230  is formed into the shape surrounded by three line segments  230   a ,  230   b  and  230   c  and an arc  230   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  230   b  and  230   c  and an angle made by the line segments  230   a  and  230   b ) are about 90° and about 1350. The fact that the shape of each of the peripheral-portion honeycomb fired bodies  220  and  230  is formed into each of these shapes is also one reason why no honeycomb fired body having an extremely small cross-sectional area is located in the peripheral portion of the honeycomb structure  200 . 
     Here, also in the honeycomb structure  200 , the cross-sectional area of center-portion honeycomb fired body  210  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     The reason for this is the same as mentioned in the first embodiment of the first aspect of the present invention. 
     The following description will discuss a method for manufacturing the honeycomb structure of the present embodiment. The method for manufacturing the honeycomb structure of the present embodiment is the same as the method for manufacturing the honeycomb structure of the first embodiment of the first aspect of the present invention, except for the following points. 
     That is, the honeycomb structure of the present embodiment can be manufactured by using the same method as the method for manufacturing the honeycomb structure of the first embodiment of the first aspect of the present invention, except that the shapes of honeycomb molded bodies formed in the molding process (1) of the manufacturing method of the first embodiment of the first aspect of the present invention have almost the same shapes as those of the center-portion honeycomb fired body  210  and the peripheral-portion honeycomb fired bodies  220  and  230  as shown in  FIG. 6  while either one end of each of the cells is not sealed, and except that, upon carrying out the combining process (4) of the manufacturing method of the first embodiment of the first aspect of the present invention, the respective honeycomb fired bodies are combined with one another so that the center-portion honeycomb fired body  210  and the peripheral-portion honeycomb fired bodies  220  and  230  are located as shown in  FIG. 6 . 
     The honeycomb structure of the present embodiment is capable of exerting the same effects as those of the honeycomb structure of the first embodiment of the first aspect of the present invention. 
     Example 1-4 
     The following description will discuss an example that more specifically discloses the second embodiment of the first aspect of the present invention. However, the first aspect of the present invention is not intended to be limited only by this example. 
     (1) By carrying out the same method as the molding process (1) of Example 1-1, raw honeycomb molded bodies having almost the same shapes as those of the center-portion honeycomb fired body  210  and peripheral-portion honeycomb fired bodies  220  and  230  shown in  FIG. 6 , with no cells being sealed, were manufactured. 
     (2) Next, the raw honeycomb molded bodies were dried by using a microwave drying apparatus to obtain dried honeycomb molded bodies. A paste having the same composition as that of the wet mixture was then filled into predetermined cells, and the filled portions of the dried honeycomb molded bodies were dried by using a drying apparatus again. 
     (3) The dried honeycomb molded bodies were degreased at 400° C., and then fired at 2200° C. under normal pressure argon atmosphere for three hours. 
     Thus, a center-portion honeycomb fired body  210  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×200 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  220  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° (line segment  220   a =45.6 mm, line segment  220   b =26.8 mm and line segment  220   c =41.8 mm) was manufactured. 
     A peripheral-portion honeycomb fired body  230 , which had the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  210 , and also had a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  230   a =24.9 mm, line segment  230   b =24.5 mm and line segment  230   c =41.8 mm) was manufactured. Here, the cross-sectional area of the center-portion honeycomb fired body  210  was 1190 mm 2 , the cross-sectional area of the peripheral-portion honeycomb fired body  220  was 1226 mm 2  and the cross-sectional area of the peripheral-portion honeycomb fired body  230  was 1226 mm 2 . Therefore, the cross-sectional area of the peripheral-portion honeycomb fired body  220  was 1.03 times larger than the cross-sectional area of the center-portion honeycomb fired body  210  and the cross-sectional area of the peripheral-portion honeycomb fired body  230  was 1.03 times larger than the cross-sectional area of the center-portion honeycomb fired body  210 . 
     (4) An adhesive paste was applied to a predetermined side face of each of the center-portion honeycomb fired body  210  and the peripheral-portion honeycomb fired bodies  220  and  230 , and nine pieces of the center-portion honeycomb fired bodies  210 , eight pieces of the peripheral-portion honeycomb fired bodies  220 , and eight pieces of the peripheral-portion honeycomb fired bodies  230  were bonded to one another with the adhesive paste interposed therebetween so as to be arranged as shown in  FIG. 6 . The adhesive paste was solidified at 180° C. in 20 minutes to manufacture a round pillar-shaped ceramic block  203  having the adhesive layer 1 mm in thickness. 
     Here, as the adhesive paste, the same adhesive paste as that used in Example 1-1 was used. 
     (5) By using a coating material paste having the same composition as the adhesive paste used in the process (4), a coating material paste layer was formed on the periphery of the ceramic block  203 . Thereafter, the coating material paste layer was dried at 120° C. to manufacture a round pillar-shaped honeycomb structure  200  having a size of 203.2 mm in diameter×200 mm in length with a coat layer  202  formed on the periphery thereof. 
     The honeycomb structure manufactured in Example 1-4 has a cross-sectional shape as shown in  FIG. 6 . 
     A regenerating process was carried out on the honeycomb structure manufactured in Example 1-4 and a regenerating rate was measured based on weight differences, by the same method as in Example 1-1 except that a 4 L engine was used instead of a 2 L engine. 
     Consequently, the regenerating rate of the honeycomb structure of Example 1-4 was 82%. 
     Third Embodiment of First Aspect of the Present Invention 
     In the methods for manufacturing the honeycomb structure according to the first and second embodiments of the first aspect of the present invention, the honeycomb structure is manufactured by forming the honeycomb fired body molded in the predetermined shape. However, the honeycomb structure according to an embodiment of the first aspect of the present invention may be manufactured according to the method mentioned below. 
     Hereinafter, another method for manufacturing a honeycomb structure according to an embodiment of the first aspect of the present invention will be described by exemplifying the case of manufacturing the honeycomb structure according to the first embodiment. 
       FIGS. 7A and 7B  are cross-sectional views for describing one example of a method for manufacturing a honeycomb structure according to the third embodiment of the first aspect of the present invention. 
     (1) Honeycomb fired bodies with either one end of each of the cells sealed are manufactured by the same method as in the processes (1) to (3) of the first embodiment of the first aspect of the present invention. 
     At this time, a center-portion honeycomb fired body  310  having a rectangular cross-sectional shape and a peripheral-portion honeycomb fired body  320 ′ having a trapezoid cross-sectional shape are manufactured (see  FIG. 7A ). 
     (2) Next, in the same manner as in the process (4) of the first embodiment, the center-portion honeycomb fired bodies  310  and the peripheral-portion honeycomb fired bodies  320 ′ are combined with one another with the adhesive paste layer interposed therebetween so as to be arranged as shown in  FIG. 7A . Moreover, the adhesive paste layer is solidified to manufacture an aggregated body of the honeycomb fired bodies  303 ′. 
     (3) Next, a periphery cutting process is carried out in which the side face of the aggregated body of the honeycomb fired bodies  303 ′ is cut by using a diamond cutter or the like to form a substantially round pillar shape so as to manufacture a ceramic block  303  in which the center-portion honeycomb fired bodies  310  and the peripheral-portion honeycomb fired bodies  320  are combined with one another with the adhesive layer  301  interposed therebetween (see  FIG. 7B ). 
     Then, if needed, a coat layer (not illustrated) is formed on the peripheral side face of the ceramic block  303  to complete a honeycomb structure. 
     Other Embodiments of First Aspect of the Present Invention 
     The cross-sectional shape of the honeycomb structure according to an embodiment of the first aspect of the present invention is not limited to a substantially round shape. The cross-sectional shape may be a substantially elliptical shape, a substantially elongated round shape, a substantially racetrack shape, or the like. 
       FIG. 8  is a cross-sectional view of a honeycomb structure according to another embodiment of the first aspect of the present invention. 
     The cross-sectional shape of the honeycomb structure illustrated in  FIG. 8  is a substantially elliptical shape. 
     A honeycomb structure  500  shown in  FIG. 8  has a structure in which a plurality of center-portion honeycomb fired bodies  510  and pluralities of peripheral-portion honeycomb fired bodies  520 ,  530  and  540  are combined with one another with an adhesive layer  501  interposed therebetween to form a ceramic block  503 . Moreover, a coat layer  502  is formed on the periphery of the ceramic block  503 . 
     The center-portion honeycomb fired body  510  has a substantially square cross-sectional shape. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  520  is formed into a shape surrounded by three line segments  520   a ,  520   b  and  520   c  and an elliptical arc  520   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  520   a  and  520   b  and an angle made by the line segments  520   b  and  520   c ) are about 90°. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  530  is formed into a shape surrounded by three line segments  530   a ,  530   b  and  530   c  and an elliptical arc  530   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  530   b  and  530   c  and an angle made by the line segments  530   a  and  530   b ) are about 90° and about 135°. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  540  is formed into a shape surrounded by three line segments  540   a ,  540   b  and  540   c  and an elliptical arc  540   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  540   a  and  540   b  and an angle made by the line segments  540   b  and  540   c ) are about 135°. 
     The center-portion honeycomb fired body  510  is the same as the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment. 
     The peripheral-portion honeycomb fired bodies  520 ,  530  and  540  have the same functions as that of the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment although outer shapes of those peripheral-portion honeycomb fired bodies are different from that of the center-portion honeycomb fired body  110 . 
     The honeycomb structure  500  includes three pieces of the center-portion honeycomb fired bodies  510  combined with one another with the adhesive layer  501  interposed therebetween, two pieces of the peripheral-portion honeycomb fired bodies  520 , four pieces of the peripheral-portion honeycomb fired bodies  530  and two pieces of the peripheral-portion honeycomb fired bodies  540 . These peripheral-portion honeycomb fired bodies are located on the periphery of the three pieces of the center-portion honeycomb fired bodies  510 . These honeycomb fired bodies are combined with one another with the adhesive layer  501  interposed therebetween so that the cross section of the honeycomb structure  500  (ceramic block  503 ) is formed into a substantially elliptical shape. 
     Here, in the honeycomb structure  500 , the cross-sectional area of each of the peripheral-portion honeycomb fired bodies  520 ,  530  and  540  is at least about 0.9 times and at most about 1.3 times larger than the cross-sectional area of the center-portion honeycomb fired body  510 . 
       FIG. 9  is a cross-sectional view of a honeycomb structure according to another embodiment of the first aspect of the present invention. 
     The cross-sectional shape of the honeycomb structure illustrated in  FIG. 9  is a substantially racetrack shape. 
     A honeycomb structure  600  shown in  FIG. 9  has a structure in which a plurality of center-portion honeycomb fired bodies  610  and pluralities of peripheral-portion honeycomb fired bodies  620 ,  630  and  640  are combined with one another with an adhesive layer  601  interposed therebetween to form a ceramic block  603 . Moreover, a coat layer  602  is formed on the periphery of the ceramic block  603 . 
     The center-portion honeycomb fired body  610  has a substantially square cross-sectional shape. 
     The peripheral-portion honeycomb fired body  620  has a substantially rectangular cross-sectional shape. 
     The cross section of the peripheral-portion honeycomb fired body  630  is formed into a shape surrounded by three line segments  630   a ,  630   b  and  630   c , and a curve  630   d  formed by one straight line and an arc. The two angles made by two line segments out of these three line segments (an angle made by the line segments  630   b  and  630   c  and an angle made by the line segments  630   a  and  630   b ) are about 90° and about 135°. 
     The cross section of the peripheral-portion honeycomb fired body  640  is formed into a shape surrounded by three line segments  640   a ,  640   b  and  640   c  and an arc  640   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  640   a  and  640   b  and an angle made by the line segments  640   b  and  640   c ) are about 135°. 
     The center-portion honeycomb fired body  610  is the same as the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment. The peripheral-portion honeycomb fired bodies  620 ,  630  and  640  have the same functions as that of the center-portion honeycomb fired body  110  used for the honeycomb structure of the first embodiment although outer shapes of those peripheral-portion honeycomb fired bodies are different from that of the center-portion honeycomb fired body  110 . 
     The honeycomb structure  600  includes three pieces of the center-portion honeycomb fired bodies  610  combined with one another with adhesive layer  601  interposed therebetween, two pieces of the peripheral-portion honeycomb fired bodies  620 , four pieces of the peripheral-portion honeycomb fired bodies  630  and two pieces of the peripheral-portion honeycomb fired bodies  640 . These peripheral-portion honeycomb fired bodies are located on the periphery of the three pieces of the center-portion honeycomb fired bodies  610 . These honeycomb fired bodies are combined with one another with the adhesive layer  601  interposed therebetween so that the cross section of the honeycomb structure  600  (ceramic block  603 ) is formed into a substantially racetrack shape. 
     Here, in the honeycomb structure  600 , the cross-sectional area of each of the peripheral-portion honeycomb fired bodies  620 ,  630  and  640  is at least about 0.9 times and at most about 1.3 times larger than the cross-sectional area of the center-portion honeycomb fired body  610 . 
     As mentioned above, the honeycomb structure according to the embodiments of the first aspect of the present invention may have a substantially elliptical cross-sectional shape as shown in  FIG. 8  or may have a substantially racetrack cross-sectional shape as shown in  FIG. 9 . 
     Moreover, in the honeycomb structure according to an embodiment of the first aspect of the present invention, the number of the center-portion honeycomb fired bodies is not limited to plural but may be one. 
     More specifically, the honeycomb structure may have a cross-sectional shape as shown in  FIG. 10 . 
       FIG. 10  is a cross-sectional view of a honeycomb structure according to another embodiment of the first aspect of the present invention. 
     The honeycomb structure  700  as illustrated in  FIG. 10  has the same structure as that of the honeycomb structure  100  of the first embodiment, except that the number of the center-portion honeycomb fired bodies is different. 
     That is, the honeycomb structure  700  as illustrated in  FIG. 10  includes one center-portion honeycomb fired body  710 , instead of four pieces of the honeycomb fired bodies  110  combined with one another with the adhesive layer  101  interposed therebetween in the honeycomb structure  100  as illustrated in  FIG. 1 . 
     Compared with the center-portion honeycomb fired body  110 , the center-portion honeycomb fired body  710  has a larger cross-sectional area but has the same functions. 
     The cross section of the peripheral-portion honeycomb fired body  720  in the honeycomb structure  700  is formed into a shape surrounded by three line segments  720   a ,  720   b  and  720   c , and an arc  720   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  720   b  and  720   c  and an angle made by the line segments  720   a  and  720   b ) are about 90° and about 135°. 
     Here, the cross-sectional area of the peripheral-portion honeycomb fired body  720  is at least about 0.9 times and at most about 1.3 times larger than that of the center-portion honeycomb fired body  710 . 
     The honeycomb structure  700  of such an embodiment is allowed to exert the same effects as those of the honeycomb structure of the first embodiment of the first aspect of the present invention. 
     Here, in  FIG. 10 , a reference numeral  701  represents an adhesive layer; a reference numeral  702  represents a coat layer; and a reference numeral  703  represents a ceramic block. 
     In the honeycomb structure of the embodiments of the first aspect of the present invention having a substantially round cross-sectional shape, four or five pieces of the honeycomb fired bodies are preferably penetrated by one diameter in the cross section of the honeycomb structure as well as another diameter that is orthogonal to the one diameter. The honeycomb structures having such structure are suitably allowed to exert the effects of the present invention. 
     Upon counting the number of the honeycomb fired bodies penetrated by the one diameter or the another diameter, if at least one of the one diameter and the another diameter is entirely superposed on or partly overlaps with an adhesive layer, one piece of honeycomb fired body located on one side adjacent to the adhesive layer is counted as one piece of honeycomb fired body penetrated by the one or another diameter. 
     With respect to the honeycomb structures of the embodiments of the first aspect of the present invention explained above, in the honeycomb structure of the first embodiment, four pieces of the honeycomb fired bodies are respectively superposed on the one diameter and the another diameter (see  FIG. 4 ). In the honeycomb structure of the second embodiment, five pieces of the honeycomb fired bodies are respectively superposed on the one diameter and the another diameter (see  FIG. 6 ). In the honeycomb structure of the embodiment shown in  FIG. 10 , three pieces of the honeycomb fired bodies are respectively superposed on the one diameter and the another diameter. Out of these three embodiments, the first and second embodiments are more preferable embodiments. 
     Referring to the drawings, the following description will discuss an embodiment of a honeycomb structure according to the second aspect of the present invention. 
     First Embodiment of Second Aspect of the Present Invention 
       FIG. 11  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the second aspect of the present invention. 
     The honeycomb structure  1100  shown in  FIG. 11  has a structure in which a plurality of center-portion honeycomb fired bodies  1110  and a plurality of peripheral-portion honeycomb fired bodies  1120  are combined with one another with an adhesive layer  1101  interposed therebetween to form a ceramic block  1103 . A coat layer  1102  is formed on the periphery of the ceramic block  1103 . 
     The center-portion honeycomb fired body  1110  has almost the same shape as that of the center-portion honeycomb fired body  110  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. The peripheral-portion honeycomb fired body  1120  has almost the same shape as that of the peripheral-portion honeycomb fired body  120  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. 
     In the center-portion honeycomb fired body  1110  and the peripheral-portion honeycomb fired body  1120 , either one end of each of the cells is sealed, so that the cell wall functions as a filter for capturing PM and the like. 
     As shown in  FIG. 11 , in the honeycomb structure  1100 , four pieces of the center-portion honeycomb fired bodies  1110  are located in the center portion of the cross section of the honeycomb structure  1100 , and eight pieces of the peripheral-portion honeycomb fired bodies  1120  are located on the periphery of the four pieces of the center-portion honeycomb fired bodies  1110 . These honeycomb fired bodies are combined with one another with the adhesive layer  1101  interposed therebetween so that the cross section of the honeycomb structure  1100  (ceramic block  1103 ) is formed into a substantially round shape. 
     In the honeycomb structure  1100 , provided that a FIG. (substantially round shape)  1105 , which is similar to the shape of the ceramic block  1103  in the cross section and is concentric with the shape of the ceramic block  1103  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1103  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1120  is located in the  FIG. 1105 . 
     In the case that a part of each of the peripheral-portion honeycomb fired bodies  1120  is located in the  FIG. 1105 , there is no peripheral-portion honeycomb fired body isolated from the center of the honeycomb structure  1100  (ceramic block  1103 ) by interposing the adhesive layer, so that the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion. 
     Further, in the honeycomb structure  1100 , the cross-sectional area of the center-portion honeycomb fired body  1110  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     This size of the cross-sectional area of the center-portion honeycomb fired body  1110  makes it easier to prevent cracks from occurring in the honeycomb structure  1100  upon carrying out a regenerating process on the honeycomb structure  1100 . 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the first embodiment of the first aspect of the present invention. 
     The following description will summarize the effects of the honeycomb structure of the present embodiment. 
     (1) In the honeycomb structure of the present embodiment, provided that a figure, which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, a part of each of the peripheral-portion honeycomb fired bodies is necessarily located in the figure. 
     Therefore, there is no peripheral-portion honeycomb fired body located only outside the figure, so that the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and unburned particulates tend not to remain upon carrying out a regenerating process. 
     (2) In the honeycomb structure of the present embodiment, the cross-sectional area of the center-portion honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2 . For this reason, cracks are less likely to occur in the honeycomb fired body upon carrying out a regenerating process. 
     (3) In the honeycomb fired body of the honeycomb structure of the present embodiment, either one end of each of the cells is sealed with a plug. Therefore, the honeycomb structure of the present embodiment is more likely to be suitably used as a diesel particulate filter. 
     (4) In the honeycomb structure of the present embodiment, since the coat layer is formed on the peripheral side face of the ceramic block, it is easier to prevent leakage of particulates from the peripheral side face of the honeycomb structure. 
     Example 2-1 
     The following description will discuss an example that specifically discloses the first embodiment of the second aspect of the present invention. Here, the second aspect of the present invention is not limited to the example. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-1. 
     Thus, a center-portion honeycomb fired body  1110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  1120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  1110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  1120   a =20.8 mm, line segment  1120   b =35.0 mm and line segment  1120   c =35.7 mm) was manufactured. 
     (2) A honeycomb structure  1100  with a coat layer  1102  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-1. 
     The honeycomb structure  1100  has a round pillar shape with a size of 143.8 mm in diameter×150 mm in length. 
     The cross-sectional shape of the honeycomb structure  1100  manufactured in Example 2-1 is shown in  FIG. 12 . 
     In the honeycomb structure  1100 , provided that a  FIG. 1105 , which is similar to the shape of the ceramic block  1103  in the cross section and is concentric with the shape of the ceramic block  1103  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1103  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1120  is necessarily located in the  FIG. 1105  (see  FIG. 12 ). 
     Comparative Example 2-1 
     A honeycomb structure same as that in Comparative Example 1-1 was manufactured. 
     The cross-sectional shape of the honeycomb structure  1400  manufactured in Comparative Example 2-1 is shown in  FIG. 13 . 
       FIG. 13  is a cross-sectional view that shows the honeycomb structure  1400  manufactured in Comparative Example 2-1, and in  FIG. 13 , a reference numeral  1410  represents a center-portion honeycomb fired body, reference numerals  1420  and  1430  represent peripheral-portion honeycomb fired bodies, a reference numeral  1401  represents an adhesive layer, a reference numeral  1402  represents a coat layer and a reference numeral  1403  represents a ceramic block. 
     In the honeycomb structure  1400 , provided that a  FIG. 1405 , which is similar to the shape of the ceramic block  1403  in the cross section and is concentric with the shape of the ceramic block  1403  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1403  in the cross section, the peripheral-portion honeycomb fired body  1430  is located only outside the  FIG. 1405 . 
     (Evaluation of Honeycomb Structure) 
     Evaluated in the same manner as in Example 1-1, the regenerating rate of the honeycomb structure of Example 2-1 was 85%, and the regenerating rate of the honeycomb structure of Comparative Example 2-1 was 70%. 
     The reason of this is presumably because a large amount of unburned particulates remained upon carrying out the regenerating process in the honeycomb structure of Comparative Example 2-1. 
     Second Embodiment of Second Aspect of the Present Invention 
       FIG. 14  is a cross-sectional view of a honeycomb structure according to the second embodiment of the second aspect of the present invention. 
     As shown in  FIG. 14 , the honeycomb structure  1200  of the present embodiment has a structure in which a plurality of center-portion honeycomb fired bodies  1210  and pluralities of peripheral-portion honeycomb fired bodies  1220  and  1230  are combined with one another with an adhesive layer  1201  interposed therebetween to form a ceramic block  1203 . A coat layer  1202  is formed on the periphery of the ceramic block  1203 . 
     The center-portion honeycomb fired body  1210  has almost the same shape as that of the center-portion honeycomb fired body  210  of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and includes the same material as that thereof. The peripheral-portion honeycomb fired bodies  1220  and  1230  have almost the same shapes as those of the peripheral-portion honeycomb fired bodies  220  and  230  of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and include the same material as those thereof. 
     As shown in  FIG. 14 , in the honeycomb structure  1200 , nine pieces of the center-portion honeycomb fired bodies  1210  are located in the center portion of the cross section of the honeycomb structure  1200 , and eight pieces of the peripheral-portion honeycomb fired bodies  1220  and eight pieces of the peripheral-portion honeycomb fired bodies  1230  are located on the periphery of the nine pieces of center-portion honeycomb fired bodies  1210 . These honeycomb fired bodies are combined with one another with the adhesive layer  1201  interposed therebetween so that the cross section of the honeycomb structure  1200  (ceramic block  1203 ) is formed into a substantially round shape. 
     In the honeycomb structure  1200 , provided that a FIG. (substantially round shape)  1205 , which is similar to the shape of the ceramic block  1203  in the cross section and is concentric with the shape of the ceramic block  1203  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1203  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1220  and  1230  is located in the  FIG. 1205 . 
     In the case that a part of each of the peripheral-portion honeycomb fired bodies  1220  and  1230  is located in the  FIG. 1205 , there is no peripheral-portion honeycomb fired body isolated from the center of the honeycomb structure  1200  (ceramic block  1203 ) by interposing the adhesive layer, so that the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion. 
     Here, also in the honeycomb structure  1200 , a cross-sectional area of the center-portion honeycomb fired body  1210  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the second embodiment of the first aspect of the present invention. 
     The honeycomb structure of the present embodiment is allowed to exert the same effects as those of the honeycomb structure of the first embodiment of the second aspect of the present invention. 
     Example 2-2 
     The following description will discuss an example that specifically discloses the second embodiment of the second aspect of the present invention. Here, the second aspect of the present invention is not limited to the example. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-4. 
     Thus, a center-portion honeycomb fired body  1210  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  1220  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  1210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 900 (line segment  1220   a =45.6 mm, line segment  1220   b =26.8 mm and line segment  1220   c =41.8 mm) was manufactured. 
     Further, a peripheral-portion honeycomb fired body  1230  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  1210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  1230   a =24.9 mm, line segment  1230   b =24.5 mm and line segment  1230   c =41.8 mm) was manufactured. 
     (2) A honeycomb structure  1200  with a coat layer  1202  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-4. 
     The honeycomb structure  1200  has a round pillar shape with a size of 203.2 mm in diameter×150 mm in length. 
     The cross-sectional shape of the honeycomb structure  1200  manufactured in Example 2-2 is shown in  FIG. 14 . 
     In the honeycomb structure  1200 , provided that a  FIG. 1205 , which is similar to the shape of the ceramic block  1203  in the cross section and is concentric with the shape of the ceramic block  1203  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1203  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1220  and  1230  is necessarily located in the  FIG. 1205  (see  FIG. 14 ). 
     Evaluated in the same manner as in Example 1-4, regenerating rate of the honeycomb structure of Example 2-2 was 82%. 
     Other Embodiments of Second Aspect of the Present Invention 
     The honeycomb structure in each of the first and second embodiments of the second aspect of the present invention may be manufactured in the same manner as in, for example, the third embodiment of the first aspect of the present invention. 
     In the honeycomb structure according to the embodiments of the second aspect of the present invention, each of the peripheral-portion honeycomb fired bodies does not necessarily have the same cross-sectional shape. 
     That is, in the embodiments of the second aspect of the present invention, in the case that a figure, which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, each of the peripheral-portion honeycomb fired bodies does not necessarily have the same cross-sectional shape as long as a part of each of the peripheral-portion honeycomb fired bodies is located in the figure. 
     Specifically, the honeycomb structure may have a cross-sectional shape shown in  FIGS. 15A and 15B . 
     Each of  FIGS. 15A and 15B  is a cross-sectional view of the honeycomb structure according to another embodiment of the second aspect of the present invention. 
     The honeycomb structure  1500  shown in  FIG. 15A  is identical to the honeycomb structure  1100  according to the first embodiment of the second aspect of the present invention except that the cross-sectional shape of the peripheral-portion honeycomb fired bodies  1520  is not the same as that of the peripheral-portion honeycomb fired bodies  1530 . 
     That is, in the honeycomb structure  1500  shown in  FIG. 15A , four pieces of the center-portion honeycomb fired bodies  1510  are combined with one another with the adhesive layer  1501  interposed therebetween, and four pieces of the peripheral-portion honeycomb fired bodies  1520  and four pieces of the peripheral-portion honeycomb fired bodies  1530  are located on the periphery of the four pieces of the center-portion honeycomb fired bodies  1510 . These honeycomb fired bodies are combined with the adhesive layer  1501  interposed therebetween to form the ceramic block  1503 . 
     The coat layer  1502  is formed on the periphery of the ceramic block  1503 . 
     The cross section of each of the peripheral-portion honeycomb fired bodies  1520  is formed into a shape surrounded by two line segments  1520   a  and  1520   b  and an arc  1520   c . An angle made by two line segments (the angle made by the line segments  1520   a  and  1520   b ) is about 90°. 
     The cross section of each of the peripheral-portion honeycomb fired bodies  1530  is formed into a shape surrounded by three line segments  1530   a ,  1530   b  and  1530   c  and an arc  1530   d . The two angles made by two line segments out of these three line segments (an angle made by the line segments  1530   b  and  1530   c  and an angle made by the line segments  1530   a  and  1530   b ) are about 90°. 
     In the honeycomb structure  1500 , provided that a figure (substantially round shape)  1505 , which is similar to the shape of the ceramic block  1503  in the cross section and is concentric with the shape of the ceramic block  1503  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1503  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1520  and  1530  is located in the  FIG. 1505 . 
     Thus, the honeycomb structure  1500  according to the embodiment of this kind is also allowed to exert the same effects as those of the honeycomb structure according to the first embodiment of the second aspect of the present invention. 
     The peripheral-portion honeycomb fired bodies  1520  and  1530  have the same functions as that of the peripheral-portion honeycomb fired bodies  1120  of the honeycomb structure  1100  although their cross-sectional shapes are different from that of the peripheral-portion honeycomb fired bodies  1120  of the honeycomb structure  1100 . 
     The honeycomb structure  1600  shown in  FIG. 15B  is identical to the honeycomb structure  1500  shown in  FIG. 15A  except for the arrangement of the peripheral-portion honeycomb fired bodies  1620  and  1630 . 
     That is, in the honeycomb structure  1600  shown in  FIG. 15B , the peripheral-portion honeycomb fired body  1620  and the peripheral-portion honeycomb fired body  1630  are alternately placed with the adhesive layer  1601  interposed therebetween, which is different from the case in the honeycomb structure  1500  shown in  FIG. 15A . 
     Each of the peripheral-portion honeycomb fired bodies  1620  and  1630  is identical to each of the peripheral-portion honeycomb fired bodies  1520  and  1530 , respectively, except for the place in the honeycomb structure. 
     In the honeycomb structure  1600 , provided that a figure (substantially round shape)  1605 , which is similar to the shape of the ceramic block  1603  in the cross section and is concentric with the shape of the ceramic block  1603  in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block  1603  in the cross section, a part of each of the peripheral-portion honeycomb fired bodies  1620  and  1630  is located in the  FIG. 1605 . 
     Thus, the honeycomb structure  1600  according to the embodiment of this kind is also allowed to exert the same effects as those of the honeycomb structure according to the first embodiment of the second aspect of the present invention. 
     In  FIG. 15B , a reference numeral  1602  represents a coat layer, and a reference numeral  1610  represents a center-portion honeycomb fired body. 
     Each of the honeycomb structures  1500  and  1600  shown in  FIGS. 15A and 15B , respectively, can be manufactured in the following manner: a necessary number of two kinds of honeycomb fired bodies with each kind having a different cross-sectional shape are combined with one another with an adhesive layer interposed therebetween to manufacture an aggregated body of the honeycomb fired bodies; and the periphery thereof is cut to manufacture the honeycomb structure. 
     This will be described more specifically by exemplifying the case of manufacturing the honeycomb structure  1500  referring to  FIGS. 16A and 16B . 
       FIGS. 16A and 16B  are cross-sectional views for describing another example of a method for manufacturing a honeycomb structure according to the embodiments of the second aspect of the present invention. 
     (1) Honeycomb fired bodies with either one end of each of the cells sealed are manufactured by the same method as in the processes (1) to (3) of the first embodiment of the first aspect of the present invention. 
     At this time, a center-portion honeycomb fired body  1510  having a substantially rectangular cross-sectional shape and peripheral-portion honeycomb fired bodies  1520 ′ and  1530 ′ having a substantially rectangular cross-sectional shape are manufactured (see  FIG. 16A ). 
     The center-portion honeycomb fired body  1510  and the peripheral-portion honeycomb fired body  1530 ′ are substantially the same honeycomb fired body. 
     (2) Next, in the same manner as in the process (4) of the first embodiment of the first aspect of the present invention, the center-portion honeycomb fired bodies  1510  and the peripheral-portion honeycomb fired bodies  1520 ′ and  1530 ′ are combined with one another with the adhesive paste layer interposed therebetween so as to be arranged as shown in  FIG. 16A . Moreover, an aggregated body of the honeycomb fired bodies  1503 ′ is manufactured by solidifying the adhesive paste layer. 
     (3) Next, a periphery cutting process is carried out in which the side face of the aggregated body of honeycomb fired bodies  1503 ′ is cut by using a diamond cutter or the like to form a substantially round pillar shape so as to manufacture a ceramic block  1503  in which the center-portion honeycomb fired bodies  1510  and the peripheral-portion honeycomb fired bodies  1520  and  1530  are combined with one another with the adhesive layer  1501  interposed therebetween (see  FIG. 16B ). 
     Then, if needed, a coat layer (not illustrated) is formed on the peripheral side face of the ceramic block  1503  to complete a honeycomb structure. 
     The shape of the honeycomb structure according to an embodiment of the second aspect of the present invention is not limited to a substantially round pillar shape. The shape may be a substantially cylindroid shape. 
     Specifically, when a figure (substantially elliptical shape), which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section and a part of each of the peripheral-portion honeycomb fired bodies is located in the figure, the honeycomb structure may be a substantially cylindroid shape having the substantially elliptical cross-sectional shape shown in  FIG. 8 . 
     This is because, also in the honeycomb structure having the substantially cylindroid shape, in the case that a part of each of the peripheral-portion honeycomb fired bodies is located in the figure, there is no peripheral-portion honeycomb fired body isolated from the center of the honeycomb structure (ceramic block) by interposing the adhesive layer, so that the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion. 
     Further, a shape of the cross section of the honeycomb structure may be a substantially elongated round shape or a substantially racetrack shape. 
     Referring to the drawings, the following description will discuss an embodiment of a honeycomb structure according to the third aspect of the present invention. 
     First Embodiment of Third Aspect of the Present Invention 
       FIG. 17  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the third aspect of the present invention. 
       FIG. 18  is an A-A line cross-sectional view of the honeycomb structure shown in  FIG. 17 . 
     The honeycomb structure  2100  shown in  FIGS. 17 and 18  has a structure in which a plurality of center-portion honeycomb fired bodies  2110  and a plurality of peripheral-portion honeycomb fired bodies  2120  are combined with one another with an adhesive layer  2101  ( 2101 A to  2101 D) interposed therebetween to form a ceramic block  2103 . A coat layer  2102  is formed on the periphery of the ceramic block  2103 . 
     The center-portion honeycomb fired body  2110  has almost the same shape as that of the center-portion honeycomb fired body  110  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. The peripheral-portion honeycomb fired body  2120  has almost the same shape as that of the peripheral-portion honeycomb fired body  120  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. 
     In the center-portion honeycomb fired body  2110  and the peripheral-portion honeycomb fired body  2120 , either one end of each of the cells is sealed, so that the cell wall functions as a filter for capturing PM and the like. 
     As shown in  FIGS. 17 and 18 , in the honeycomb structure  2100 , four pieces of the center-portion honeycomb fired bodies  2110  are located in the center portion of the cross section of the honeycomb structure  2100 , and eight pieces of the peripheral-portion honeycomb fired bodies  2120  are located on the periphery of the four pieces of the center-portion honeycomb fired bodies  2110 . These honeycomb fired bodies are combined with one another with the adhesive layers  2101  interposed therebetween so that the cross section of the honeycomb structure  2100  (ceramic block  2103 ) is formed into a substantially round shape. 
     The four pieces of the center-portion honeycomb fired bodies  2110  combined with one another by interposing the adhesive layer  2101 A therebetween form the center portion in the cross-section of the honeycomb structure  2100 . The eight pieces of the peripheral-portion honeycomb fired bodies  2120  combined with one another by interposing the adhesive layers  2101 C and  2101 D form the peripheral portion in the cross section of the honeycomb structure  2100 . 
     In the cross section of the honeycomb structure  2100  having the above-mentioned configuration (see  FIG. 18 ), the region occupied by the four pieces of the center-portion honeycomb fired bodies  2110 , the adhesive layer  2101 A combining the center-portion honeycomb fired bodies  2110  with one another, the adhesive layer  2101 B combining the center-portion honeycomb fired body  2110  with the peripheral-portion honeycomb fired bodies  2120  corresponds to the center portion, and the region occupied by the eight pieces of the peripheral-portion honeycomb fired bodies  2120 , and the adhesive layers  2101 C and  2101 D combining the peripheral-portion honeycomb fired bodies  2120  with one another corresponds to the peripheral portion. 
     Further, in the cross section of the honeycomb structure  2100 , the adhesive layer  2101 C (first peripheral-portion adhesive layer) that is formed in a direction extending from a corner point of the center portion to the peripheral side face of the honeycomb structure  2100  and the adhesive layer  2101 D (second peripheral-portion adhesive layer) that is formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure  2100  form an angle of about 45°. 
     When the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer form an angle of about 45° as mentioned above, it is easier to prevent damages from occurring in the honeycomb structure. 
     Moreover, in the honeycomb structure  2100 , at the corner point of the above-mentioned center-portion, the first peripheral-portion adhesive layer  2101 C and the adhesive layers  2101 B combining the center-portion honeycomb fired body  2110  with the peripheral-portion honeycomb fired body  2120  form a Y shape. 
     When there is a portion where the adhesive layers form a Y shape in the cross-section of the honeycomb structure as mentioned above, it is easier to prevent damages from occurring in the honeycomb structure. 
     Moreover, in the honeycomb structure  2100 , the second peripheral-portion adhesive layer  2101 D and the adhesive layers  2101 A combining the center-portion honeycomb fired bodies  2110  with one another form a substantially straight line. 
     The adhesive layer of this kind is more likely to play a role as, so as to say, a beam for improving strength of the honeycomb structure. 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the first embodiment of the first aspect of the present invention. 
     The following description will summarize the effects of the honeycomb structure of the present embodiment. 
     (1) In the honeycomb structure of the present embodiment, since the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer form an angle of about 45°, it is easier to prevent the honeycomb structure from being damaged due to compressive stress applied from the outside of the honeycomb structure. 
     (2) In the honeycomb structure of the present embodiment, since there is a portion where the adhesive layer forms a Y shape in the cross section of the honeycomb structure, it is easier to prevent the honeycomb structure from being damaged. 
     (3) In the honeycomb fired body of the honeycomb structure of the present embodiment, either one end of each of the cells is sealed with a plug. Therefore, the honeycomb structure of the present embodiment is more likely to be suitably used as a diesel particulate filter. 
     (4) In the honeycomb structure of the present embodiment, since the coat layer is formed on the peripheral side face of the ceramic block, it is easier to prevent leakage of particulates from the peripheral side face of the honeycomb structure. 
     Example 3-1 
     The following description will discuss an example that specifically discloses the first embodiment of the third aspect of the present invention. Here, the third aspect of the present invention is not limited to the example. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-1. 
     Thus, a center-portion honeycomb fired body  2110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  2120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  2110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  2120   a =20.8 mm, line segment  2120   b =35.0 mm and line segment  2120   c =35.7 mm) was manufactured. 
     (2) A honeycomb structure  2100  with a coat layer  2102  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-1. 
     The honeycomb structure  2100  has a round pillar shape with a size of 143.8 mm in diameter×150 mm in length. 
     The cross-sectional shape of the honeycomb structure  2100  manufactured in Example 3-1 is shown in  FIG. 18 . 
     In the honeycomb structure  2100 , the first peripheral-portion adhesive layer  2101 C and the second peripheral-portion adhesive layer  2101 D form an angle of 45° in the cross section of the honeycomb structure  2100 . 
     Further, in the cross section of the honeycomb structure  2100 , there is a portion where the first peripheral-portion adhesive layer  2101 C and the adhesive layers  2101 B combining the center-portion honeycomb fired body  2110  and the peripheral portion honeycomb fired body  2120  form a Y shape. 
     Comparative Example 3-1 
     A honeycomb structure same as that in Comparative Example 1-1 was manufactured. 
     The cross-sectional shape of the honeycomb structure  2400  manufactured in Comparative Example 3-1 is shown in  FIG. 19 . 
       FIG. 19  is a cross-sectional view that shows the honeycomb structure  2400  manufactured in Comparative Example 3-1, and in  FIG. 19 , a reference numeral  2410  represents a center-portion honeycomb fired body, reference numerals  2420  and  2430  represent peripheral-portion honeycomb fired bodies, reference numerals  2401 A to  2401 D represent adhesive layers, a reference numeral  2402  represents a coat layer and a reference numeral  2403  represents a ceramic block. 
     In the honeycomb structure  2400 , the first peripheral-portion adhesive layer  2401 C and the second peripheral-portion adhesive layer  2401 D are in parallel or form an angle of 90° in the cross section. 
     Further, in the cross section of the honeycomb structure  2400 , there is no portion where the adhesive layers form the Y-shape. 
     (Evaluation of Honeycomb Structure) 
     With respect to the honeycomb structure manufactured in each of Example 3-1 and Comparative example 3-1, isostatic strength was measured in conformity to “JASO M 505-87; method for testing ceramic monolith supporting carrier for automobile exhaust-gas purifying catalyst” defined in Japanese Automobile Standards Organization instituted by Society of Automotive Engineers of Japan, Inc. 
     The contents of JASO M 505-87 are incorporated herein by reference in their entirety. 
     Isostatic strength of the honeycomb structure of Example 3-1 was measured to be 9 MPa. 
     On the other hand, isostatic strength of the honeycomb structure of Comparative Example 3-1 was measured to be 6 MPa. 
     As mentioned above, it is clear that the honeycomb structure according to the first embodiment of the third aspect of the present invention was more suitable for preventing the honeycomb structure from being damaged than the conventional honeycomb structure (the honeycomb structure of the Comparative Example 3-1). 
     Second Embodiment of Third Aspect of the Present Invention 
       FIG. 20  is a cross-sectional view of a honeycomb structure according to the second embodiment of the third aspect of the present invention. 
     As shown in  FIG. 20 , the honeycomb structure  2200  of the present embodiment has a structure in which a plurality of center-portion honeycomb fired bodies  2210  and pluralities of peripheral-portion honeycomb fired bodies  2220  and  2230  are combined with one another with adhesive layers  2201 A to  2201 D interposed therebetween to form a ceramic block  2203 . A coat layer  2202  is formed on the periphery of the ceramic block  2203 . 
     The center-portion honeycomb fired body  2210  has almost the same shape as that of the center-portion honeycomb fired body  210  of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and includes the same material as that thereof. The peripheral-portion honeycomb fired bodies  2220  and  2230  have almost the same shape as those of the peripheral-portion honeycomb fired bodies  220  and  230  of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and include the same material as those thereof. 
     As shown in  FIG. 20 , in the honeycomb structure  2200 , nine pieces of the center-portion honeycomb fired bodies  2210  are located in the center portion of the cross section of the honeycomb structure  2200 , and eight pieces of the peripheral-portion honeycomb fired bodies  2220  and eight pieces of the peripheral-portion honeycomb fired bodies  2230  are located on the periphery of the nine pieces of the center-portion honeycomb fired bodies  2210 . These honeycomb fired bodies are combined with one another with the adhesive layers  2201 A to  2201 D interposed therebetween so that the cross section of the honeycomb structure  2200  (ceramic block  2203 ) is formed into a substantially round shape. 
     The nine pieces of the center-portion honeycomb fired bodies  2210  combined with one another by interposing the adhesive layer  2201 A therebetween form the center portion in the cross-section of the honeycomb structure  2200 . The total 16 pieces of the peripheral-portion honeycomb fired bodies  2220  and  2230  combined with one another by interposing the adhesive layer  2201 C or  2201 D form the peripheral portion in the cross section of the honeycomb structure  2200 . 
     In the cross section of the honeycomb structure  2200  having the above-mentioned configuration, the region occupied by the nine pieces of the center-portion honeycomb fired bodies  2210 , the adhesive layer  2201 A combining the center-portion honeycomb fired bodies  2210  with one another, and the adhesive layer  2201 B combining the center-portion honeycomb fired body  2210  with the peripheral-portion honeycomb fired bodies  2220  and  2230  corresponds to the center portion, and the region occupied by the 16 pieces of the peripheral-portion honeycomb fired bodies  2220  and  2230 , and the adhesive layers  2201 C and  2201 D combining the peripheral-portion honeycomb fired bodies  2220  and  2230  with one another corresponds to the peripheral portion. 
     Further, in the cross section of the honeycomb structure  2200 , the adhesive layer  2201 C (first peripheral-portion adhesive layer) that is formed in a direction extending from a corner point of the center portion to the peripheral side face of the honeycomb structure  2200  and the adhesive layer  2201 D (second peripheral-portion adhesive layer) that is formed in a direction extending from the center portion other than the corner points thereof to the peripheral side face of the honeycomb structure  2200  form an angle of about 45°. 
     When the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer form an angle of about 45° as mentioned above, it is easier to prevent damages from occurring in the honeycomb structure. 
     Moreover, in the honeycomb structure  2200 , at the corner point of the above-mentioned center portion, the first peripheral-portion adhesive layer  2201 C and the adhesive layers  2201 B combining the center-portion honeycomb fired body  2210  with the peripheral-portion honeycomb fired body  2220  form a Y shape. 
     When there is a portion where the adhesive layers form a Y shape in the cross-section of the honeycomb structure as mentioned above, it is easier to prevent damages from occurring in the honeycomb structure. 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the second embodiment of the first aspect of the present invention. 
     The honeycomb structure of the present embodiment is allowed to exert the same effects as those of the honeycomb structure of the first embodiment of the third aspect of the present invention. 
     Example 3-2 
     The following description will discuss an example that specifically discloses the second embodiment of the third aspect of the present invention. Here, the third aspect of the present invention is not limited to the example. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-4. 
     Thus, a center-portion honeycomb fired body  2210  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×200 mm, the number of cells (cell density) of 300 pcs/inch 2  and a thickness of cell walls of 0.25 mm (10 mil) was manufactured. 
     Also, a peripheral-portion honeycomb fired body  2220  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  2210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° (line segment  2220   a =45.6 mm, line segment  2220   b =26.8 mm and line segment  2220   c =41.8 mm) was manufactured. 
     Further, a peripheral-portion honeycomb fired body  2230  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the center-portion honeycomb fired body  2210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  2230   a =24.9 mm, line segment  2230   b =24.5 mm and line segment  2230   c =41.8 mm) was manufactured. 
     (2) A honeycomb structure  2200  with a coat layer  2202  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-4. 
     The honeycomb structure  2200  has a round pillar shape with a size of 203.2 mm in diameter×200 mm in length. 
     The cross-sectional shape of the honeycomb structure  2200  manufactured in Example 3-2 is shown in  FIG. 20 . 
     In the honeycomb structure  2200 , the first peripheral-portion adhesive layer  2201 C and the second peripheral-portion adhesive layer  2201 D form an angle of 45° in the cross section of the honeycomb structure  2200 . 
     Further, in the cross section of the honeycomb structure  2200 , there is a portion where the first peripheral-portion adhesive layer  2201 C and the peripheral-portion adhesive layers  2201 B combining the center-portion honeycomb fired body  2210  and the peripheral-portion honeycomb fired body  2220  form a Y shape. 
     Measurement of isostatic strength of the honeycomb structure manufactured in Example 3-2 was carried out in the same manner as in Example 3-1. 
     Isostatic strength of the honeycomb structure manufactured in Example 3-2 was measured to be 8.5 MPa. 
     As mentioned above, it is clear that the honeycomb structure manufactured in Example 3-2 (the second embodiment of the third aspect of the present invention) is suitable for preventing the honeycomb structure from being damaged. 
     Other Embodiments of Third Aspect of the Present Invention 
     The honeycomb structure according to each of the first and second embodiments of the third aspect of the present invention may be manufactured in the same manner as in, for example, the third embodiment of the first aspect of the present invention. 
     The cross-sectional shape of the honeycomb structure according to the embodiments of the third aspect of the present invention is not limited to a substantially round shape. The cross-sectional shape may be a substantially elliptical shape, a substantially elongated round shape, a substantially racetrack shape or the like. 
     Moreover, in the honeycomb structure according to the embodiments of the present invention, the number of the center-portion honeycomb fired body is not limited to plural, and may be one. 
     Specifically, the shape of the cross section of the honeycomb structure may be a shape shown in  FIG. 21 . 
       FIG. 21  is a cross-sectional view of a honeycomb structure according to another embodiment of the third aspect of the present invention. 
     The honeycomb structure  2700  as illustrated in  FIG. 21  has the same structure as that of the honeycomb structure  2100  of the first embodiment of the third aspect of the present invention, except that the number of the center-portion honeycomb fired bodies is different. 
     That is, the honeycomb structure  2700  as illustrated in  FIG. 21  includes one center-portion honeycomb fired body  2710  instead of the four pieces of the center-portion honeycomb fired bodies  2110  combined with one another with the adhesive layer  2101 A interposed therebetween in the honeycomb structure  2100  as illustrated in  FIG. 18 . 
     Compared with the center-portion honeycomb fired body  2110 , the center-portion honeycomb fired body  2710  has a larger cross-sectional area but has the same functions. 
     In the cross-section of the honeycomb structure  2700  of this kind, the first peripheral-portion adhesive layer  2701 C and the second peripheral-portion adhesive layer  2701 D form an angle of about 45°. 
     Further, in the honeycomb structure  2700 , the first peripheral-portion adhesive layer  2701 C and the adhesive layers  2701 B combining the center-portion honeycomb fired body  2710  with the peripheral-portion honeycomb fired body  2720  form a Y shape at a corner point of the center portion. 
     Therefore, the honeycomb structure  2700  is allowed to exert the same effects as the effects described in the first embodiment of the third aspect of the present invention. 
     Here, in  FIG. 21 , the reference numeral  2702  represents a coat layer, and the reference numeral  2703  represents a ceramic block. 
     In the cross section of the honeycomb structure according to the embodiments of the third aspect of the present invention, the angle formed by the first peripheral-portion adhesive layer and the second peripheral portion adhesive layer is not limited to about 45°, and may be an angle of at least about 40° and at most about 50°. 
     This is because, the angle formed by the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer within the above range is appropriate for preventing damages due to compressive stress generated in various directions on the peripheral side face of the honeycomb structure. 
     Although all the angles formed by the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer are angles of at least about 40° and at most about 50° in the honeycomb structure of the embodiment mentioned above, not all of the angles should be at least about 40° and at most about 50° as long as at least one angle is at least about 40° and at most about 50° out of the angles formed by the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer in the honeycomb structure of the present embodiment. 
     With respect to the honeycomb structure according to the embodiments of the third aspect of the present invention, the cross-sectional area of the center-portion honeycomb fired body is preferably at least about 900 mm 2  and at most about 2500 mm 2 . 
     In the case that the cross-sectional area of the center-portion honeycomb fired body is in the above range, cracks tend not to occur in the honeycomb structure upon carrying out a regenerating process on the honeycomb structure. 
     Referring to the drawings, the following description will discuss an embodiment of a honeycomb structure according to the fourth aspect of the present invention. 
     First Embodiment of Fourth Aspect of the Present Invention 
     In the honeycomb structure of the present embodiment, a cross-sectional area of a ceramic block is about 10000 mm 2  or more and less than 25000 mm 2 . 
       FIG. 22  is a perspective view schematically showing a honeycomb structure according to the first embodiment of the fourth aspect of the present invention. 
       FIG. 23  is an A-A line cross-sectional view of the honeycomb structure shown in  FIG. 22 . 
     The honeycomb structure  3100  shown in  FIGS. 22 and 23  has a structure in which a plurality of honeycomb fired bodies  3110  and a plurality of honeycomb fired bodies  3120  are combined with one another with an adhesive layer  3101  interposed therebetween to form a ceramic block  3103 . A coat layer  3102  is formed on the periphery of the ceramic block  3103 . 
     The honeycomb fired body  3110  has almost the same shape as that of the center-portion honeycomb fired body  110  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. The honeycomb fired body  3120  has almost the same shape as that of the peripheral-portion honeycomb fired body  120  of the honeycomb structure  100  according to the first embodiment of the first aspect of the present invention, and includes the same material as that thereof. 
     In the honeycomb fired body  3110  and the honeycomb fired body  3120 , either one end of each of the cells is sealed, so that the cell wall functions as a filter for capturing PM and the like. 
     As shown in  FIGS. 22 and 23 , in the honeycomb structure  3100 , four pieces of the honeycomb fired bodies  3110  combined with one another with the adhesive layer  3101  interposed therebetween are located in the center portion of the cross section of the honeycomb structure  3100 , and eight pieces of the honeycomb fired bodies  3120  are located on the periphery of the four pieces of the honeycomb fired bodies  3110  so that the cross section of the honeycomb structure  3100  (ceramic block  3103 ) is formed into a substantially round shape. 
     In the cross section of the honeycomb structure  3100 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3110  and  3120  and extends from the center of gravity  3103 A of the ceramic block  3103  to the periphery of the ceramic block  3103  (see an arrow in  FIG. 23 ) is two or less. 
     As mentioned above, in the case that the cross-sectional area of the ceramic block is about 10000 mm 2  or more and less than 25000 mm 2 , and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section is two or less, the honeycomb structure is allowed to exert the following effects: 
     the adhesive layer easily alleviates thermal stress, and thus, it is easier to prevent occurrence of cracks and damages on the honeycomb structure, and 
     the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion of the honeycomb structure, and thus, unburned particulates tend not to remain upon carrying out a regenerating process. 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the first embodiment of the first aspect of the present invention. 
     The following description will summarize the effects of the honeycomb structure of the present embodiment. 
     (1) In the honeycomb structure of the present embodiment, the cross-sectional area of the honeycomb fired body is at least about 900 mm 2  and at most about 2500 mm 2 , the cross-sectional area of the ceramic block is about 10000 mm 2  or more and less than 25000 mm 2 , and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section is two or less. 
     Thus, the honeycomb structure is allowed to exert the following effects:
         the adhesive layer easily alleviates thermal stress, and thus, it is easier to prevent occurrence of cracks and damages on the honeycomb structure; and       

     the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion of the honeycomb structure, and thus, unburned particulates tend not to remain upon carrying out a regenerating process. 
     (2) In the honeycomb fired body of the honeycomb structure of the present embodiment, either one end of each of the cells is sealed with a plug. Therefore, the honeycomb structure of the present embodiment is more likely to be suitably used as a diesel particulate filter. 
     (3) In the honeycomb structure of the present embodiment, since the coat layer is formed on the peripheral side face of the ceramic block, it is easier to prevent leakage of particulates from the peripheral side face of the honeycomb structure. 
     (4) In the honeycomb structure of the present embodiment, since the ceramic block has a substantially round cross-sectional shape, in the case that the cross-sectional area and the number of the adhesive layers existing on a route which extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section satisfy the above relationships, the effect that the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion is more likely to be easily exerted. 
     Example 4-1 
     The following description will discuss an example that specifically discloses the first embodiment of the fourth aspect of the present invention. Here, the fourth aspect of the present invention is not limited to the examples. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-1. 
     Thus, a honeycomb fired body  3110  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×150 mm, the number of cells (cell density) of 300 pcs/inch 2 , a thickness of cell walls of 0.25 mm (10 mil), and a cross-sectional area of 1190 mm 2  was manufactured. 
     Also, a honeycomb fired body  3120  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the honeycomb fired body  3110  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° and 135° (line segment  3120   a =20.3 mm, line segment  3120   b =34.6 mm and line segment  3120   c =34.6 mm), and a cross-sectional area of 1293 mm 2  was manufactured. 
     (2) A honeycomb structure  3100  with a coat layer  3102  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-1. The honeycomb structure  3100  has a round pillar shape with a size of 143.8 mm in diameter×150 mm in length. 
     The cross-sectional shape of the honeycomb structure manufactured in Example 4-1 is shown in  FIG. 23 . 
     The cross-sectional area of the honeycomb fired body  3110  is 1190 mm 2 , the cross-sectional area of the honeycomb fired body  3120  is 1293 mm 2 , the cross-sectional area of the ceramic block  3103  is 16151 mm 2 , and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3110  and  3120  and extends from the center of gravity  3103 A of the ceramic block  3103  to the periphery of the ceramic block  3103  in the cross section is two. 
     Comparative Example 4-1 
     A honeycomb structure same as that in Comparative Example 1-1 was manufactured. 
     The cross-sectional shape of the honeycomb structure manufactured in Comparative Example 4-1 is shown in  FIG. 24 . 
       FIG. 24  is a cross-sectional view that shows the honeycomb structure  3400  manufactured in Comparative Example 4-1, and in  FIG. 24 , reference numerals  3410 ,  3420 , and  3430  represent honeycomb fired bodies, a reference numeral  3401  represents an adhesive layer, a reference numeral  3402  represents a coat layer and a reference numeral  3403  represents a ceramic block. 
     The cross-sectional area of the honeycomb fired body  3410  is 1190 mm 2 , the cross-sectional area of the honeycomb fired body  3420  is 1095 mm 2 , the cross-sectional area of the honeycomb fired body  3430  is 357 mm 2 , the cross-sectional area of the ceramic block  3403  is 16151 mm 2 , and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3410 ,  3420 , and  3430  and extends from the center of gravity  3403 A of the ceramic block  3403  to the periphery of the ceramic block  3403  in the cross section is three, while the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3410  and  3420  and extends from the center of gravity  3403 A of the ceramic block  3403  to the periphery of the ceramic block  3403  in the cross section is two. 
     (Evaluation of Honeycomb Structure) 
     Evaluated in the same manner as in Example 1-1, the regenerating rate of the honeycomb structure of Example 4-1 was 85%, and the regenerating rate of the honeycomb structure of Comparative Example 4-1 was 70%. 
     The reason of this is presumably because a large amount of unburned particulates remained upon carrying out the regenerating process on the honeycomb structure of Comparative Example 4-1. 
     Second Embodiment of Fourth Aspect of the Present Invention 
       FIG. 25  is a cross-sectional view of a honeycomb structure according to the second embodiment of the fourth aspect of the present invention. 
     In the honeycomb structure  3200  of the present embodiment, a cross-sectional area of a ceramic block  3203  is 25000 mm 2  or more and less than 40000 mm 2 . 
     As shown in  FIG. 25 , the honeycomb structure  3200  of the present embodiment has a structure in which pluralities of honeycomb fired bodies  3210 ,  3220  and  3230  are combined with one another with an adhesive layer  3201  interposed therebetween to form a ceramic block  3203 . A coat layer  3202  is formed on the periphery of the ceramic block  3203 . 
     The honeycomb fired bodies  3210  has almost the same shape as that of the center-portion honeycomb fired bodies  210  of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and includes the same material as that thereof. The honeycomb fired bodies  3220  and  3230  have almost the same shape as those of the peripheral-portion honeycomb fired bodies  220  and  230  respectively of the honeycomb structure  200  according to the second embodiment of the first aspect of the present invention, and include the same material as those thereof. 
     Further, a cross-sectional area of each of the honeycomb fired bodies  3210 ,  3220  and  3230  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     As shown in  FIG. 25 , in the honeycomb structure  3200 , nine pieces of the honeycomb fired bodies  3210  combined with one another with the adhesive layer  3201  interposed therebetween are located in the center portion of the cross section of the honeycomb structure  3200 , and eight pieces of the honeycomb fired bodies  3220  and eight pieces of the honeycomb fired bodies  3230  are located on the periphery of the nine pieces of the honeycomb fired bodies  3210  so that the cross section of the honeycomb structure  3200  (ceramic block  3203 ) is formed into a substantially round shape. 
     In the cross section of the honeycomb structure  3200 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3210  and  3220  and extends from the center of gravity  3203 A of the ceramic block  3203  to the periphery of the ceramic block  3203  (see an arrow in  FIG. 25 ) is two. 
     In the cross section of the honeycomb structure  3200 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3210  and  3230  and extends from the center of gravity  3203 A of the ceramic block  3203  to the periphery of the ceramic block  3203  (see an arrow in  FIG. 25 ) is three. 
     As mentioned above, in the case that the cross-sectional area of the ceramic block is 25000 mm 2  or more and less than 40000 mm 2 , and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section is three or less, the honeycomb structure is allowed to exert the following effects: 
     the adhesive layer easily alleviates thermal stress, and thus, it is easier to prevent occurrence of cracks and damages on the honeycomb structure; and 
     the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and thus, unburned particulates tend not to remain upon carrying out a regenerating process. 
     The honeycomb structure according to the present embodiment can be manufactured by the same method for manufacturing the honeycomb structure according to the second embodiment of the first aspect of the present invention. 
     The honeycomb structure of the present embodiment is allowed to exert the same effects as those of the honeycomb structure of the first embodiment of the fourth aspect of the present invention. 
     Example 4-2 
     The following description will discuss an example that specifically discloses the second embodiment of the fourth aspect of the present invention. Here, the fourth aspect of the present invention is not limited to the example. 
     (1) Honeycomb fired bodies were manufactured in the same manner as in the processes (1) to (3) of Example 1-4. 
     Thus, a honeycomb fired body  3210  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×200 mm, the number of cells (cell density) of 300 pcs/inch 2 , a thickness of cell walls of 0.25 mm (10 mil), and a cross-sectional area of 1190 mm 2  was manufactured. 
     Also, a honeycomb fired body  3220  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the honeycomb fired body  3210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 90° (line segment  3220   a =45.6 mm, line segment  3220   b =26.8 mm and line segment  3220   c =41.8 mm), and a cross-sectional area of 1226 mm 2  was manufactured. 
     Further, a honeycomb fired body  3230  having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the honeycomb fired body  3210  and also having a cross-sectional shape surrounded by three line segments and an arc, with the two angles, made by two line segments out of these three line segments, being 900 and 135° (line segment  3230   a =24.9 mm, line segment  3230   b =24.5 mm and line segment  3230   c =41.8 mm), and a cross-sectional area of 1226 mm 2  was manufactured. 
     (2) A honeycomb structure  3200  with a coat layer  3202  formed on the periphery thereof was manufactured in the same manner as in the processes (4) and (5) of Example 1-4. 
     In the honeycomb structure  3200 , a cross-sectional area of the ceramic block is 32302 mm 2 . The honeycomb structure  3200  has a round pillar shape with a size of 203.2 mm in diameter×200 mm in length. 
     The cross-sectional shape of the honeycomb structure manufactured in Example 4-2 is shown in  FIG. 25 . 
     The cross-sectional area of the honeycomb fired body  3210  is 1190 mm 2 , the cross-sectional area of the honeycomb fired body  3220  is 1226 mm 2 , the cross-sectional area of the honeycomb fired body  3230  is 1226 mm 2 , the cross-sectional area of the ceramic block  3203  is 32302 mm 2 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3210  and  3220  and extends from the center of gravity  3203 A of the ceramic block  3203  to the periphery of the ceramic block  3203  in the cross section is two, and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3210  and  3230  and extends from the center of gravity  3203 A of the ceramic block  3203  to the periphery of the ceramic block  3203  in the cross section is three. 
     Comparative Example 4-2 
     (1) By carrying out the same process as the process (1) of Example 4-1, a honeycomb fired body including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×200 mm, the number of cells (cell density) of 300 pcs/inch 2 , a thickness of cell walls of 0.25 mm (10 mil), and a cross-sectional area of 1190 mm 2  was manufactured. 
     (2) An adhesive paste was applied to a predetermined side face of the honeycomb fired body, and 32 pieces of the honeycomb fired bodies were bonded to one another with the adhesive paste interposed therebetween. The adhesive paste was solidified at 180° C. in 20 minutes to manufacture an aggregated body of the honeycomb fired bodies having a rectangular pillar shape, with the thickness of the adhesive layer being 1 mm. 
     Here, as the adhesive paste, the same adhesive paste as that used in Example 1-1 was used. 
     (3) Next, the periphery of the aggregated body of the honeycomb fired bodies was cut by using a diamond cutter to manufacture a round pillar-shaped ceramic block having a cross-sectional area of 32302 mm 2 . 
     Subsequently, a coating material paste layer was formed on the periphery of the ceramic block by using the coating material paste made of the same material as that of the adhesive paste. Further, this coating material paste layer was dried at a temperature of 120° C. to manufacture a round pillar-shaped honeycomb structure having a size of 203.2 mm in diameter×200 mm in length, with a coat layer formed on the periphery thereof. 
     The cross-sectional shape of the honeycomb structure manufactured in Comparative Example 4-2 is shown in  FIG. 26 . 
       FIG. 26  is a cross-sectional view that shows the honeycomb structure  4400  manufactured in Comparative Example 4-2, and in  FIG. 26 , reference numerals  4410 ,  4420 , and  4430  represent honeycomb fired bodies, a reference numeral  4401  represents an adhesive layer, a reference numeral  4402  represents a coat layer and a reference numeral  4403  represents a ceramic block. 
     The cross-sectional area of the ceramic block  4403  is 32302 mm 2 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  4410  and  4420  and extends from the center of gravity  4403 A of the ceramic block  4403  to the periphery of the ceramic block  4403  in the cross section is three, and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  4410  and  4430  and extends from the center of gravity  4403 A of the ceramic block  4403  to the periphery of the ceramic block  4403  in the cross section is four. 
     Evaluated in the same manner as in Example 1-4, the regenerating rate of the honeycomb structure of Example 4-2 was 82%, and the regenerating rate of the honeycomb structure of Comparative Example 4-2 was 65%. 
     Third Embodiment of Fourth Aspect of the Present Invention 
       FIG. 27  is a cross-sectional view of a honeycomb structure according to the third embodiment of the fourth aspect of the present invention. 
     In the honeycomb structure  3300  of the present embodiment, a cross-sectional area of a ceramic block  3303  is 40000 mm 2  or more and about 55000 mm 2  or less. 
     As shown in  FIG. 27 , the honeycomb structure  3300  of the present embodiment has a structure in which pluralities of honeycomb fired bodies  3310 ,  3320 ,  3330  and  3340  are combined with one another with an adhesive layer  3301  interposed therebetween to form a ceramic block  3303 . A coat layer  3302  is formed on the periphery of the ceramic block  3303 . 
     The cross section of each of the honeycomb fired bodies  3310  and  3320  has a substantially square shape. 
     The cross section of the honeycomb fired body  3330  has a shape surrounded by four line segments  3330   a ,  3330   b ,  3330   c , and  3330   d  and one arc  3330   e , and all angles formed by two line segments of the four line segments (an angle formed by the line segments  3330   a  and  3330   b , an angle formed by the line segments  3330   b  and  3330   c , and an angle formed by the line segments  3330   c  and  3330   d ) are about 90°. 
     The cross section of the honeycomb fired body  3340  has a shape surrounded by two line segments  3340   a  and  3340   b  and one arc  3340   c , and the angle formed by the two line segments (the angle formed by the line segments  3340   a  and  3340   b ) is about 45°. 
     That is, the honeycomb fired bodies  3310  and  3320  are the same as the honeycomb fired body  3110  used for the honeycomb structure according to the first embodiment of the fourth aspect of the present invention. The honeycomb fired bodies  3330  and  3340  have the same functions as that of the honeycomb fired body  3110  of the honeycomb structure according to the first embodiment of the fourth aspect of the present invention, although the outer shapes of the honeycomb fired bodies  3330  and  3340  are different from that of the honeycomb fired body  3110 . 
     The cross-sectional area of each of the honeycomb fired bodies  3310 ,  3320 ,  3330 , and  3340  is at least about 900 mm 2  and at most about 2500 mm 2 . 
     Further, the honeycomb fired bodies  3310 ,  3320 ,  3330 , and  3340  include a porous silicon carbide sintered body. 
     As shown in  FIG. 27 , in the honeycomb structure  3300 ,  21  pieces of the honeycomb fired bodies  3310  combined with one another with the adhesive layer  3301  interposed therebetween are located near the center of the cross section of the honeycomb structure  3300 , and four pieces of the honeycomb fired bodies  3320 , eight pieces of the honeycomb fired bodies  3330 , and eight pieces of the honeycomb fired bodies  3340  are located on the periphery of the 21 pieces of the honeycomb fired bodies  3310 . These honeycomb fired bodies are combined with one another with the adhesive layer  3301  interposed therebetween so that the cross section of the ceramic block  3303  is formed into a substantially round shape. 
     In the cross section of the honeycomb structure  3300 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3320  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  (see an arrow in  FIG. 27 ) is three. 
     In the cross section of the honeycomb structure  3300 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3330  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  (see an arrow in  FIG. 27 ) is four. 
     In the cross section of the honeycomb structure  3300 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3340  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  (see an arrow in  FIG. 27 ) is four. 
     As mentioned above, in the case that the cross-sectional area of the ceramic block is 40000 mm 2  or more and about 55000 mm 2  or less, and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies and extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section is four or less, the honeycomb structure is allowed to exert the following effects: 
     the adhesive layer easily alleviates thermal stress, and thus, it is easier to prevent occurrence of cracks and damages on the honeycomb structure, and 
     the honeycomb structure tends not to have a temperature distribution between the center portion and the peripheral portion, and thus, unburned particulates tend not to remain upon carrying out a regenerating process. 
     The following description will discuss a method for manufacturing a honeycomb structure of the present embodiment. 
       FIGS. 28A and 28B  are cross-sectional views for describing an example of a method for manufacturing a honeycomb structure according to the third embodiment of the fourth aspect of the present invention. 
     (1) Honeycomb fired bodies with either one end of each of the cells sealed are manufactured by the same method as in the processes (1) to (3) of the first embodiment of the first aspect of the present invention. 
     At this time, a honeycomb fired body  3610  having a square cross-sectional shape and a honeycomb fired body  3640 ′ having a trapezoid cross-sectional shape are manufactured (see  FIG. 28A ). 
     (2) Next, in the same manner as in the process (4) of the first embodiment of the first aspect of the present invention, the honeycomb fired bodies  3610  and the honeycomb fired bodies  3640 ′ are combined with one another with the adhesive paste layer interposed therebetween so as to be arranged as shown in  FIG. 28A . Further, the adhesive paste layer is solidified to manufacture an aggregated body of the honeycomb fired bodies  3603 ′. 
     (3) Next, a periphery cutting process is carried out in which the side face of the aggregated body of the honeycomb fired bodies  3603 ′ is cut by using a diamond cutter or the like to form a substantially round pillar shape so as to manufacture a ceramic block  3603  in which the honeycomb fired bodies  3610 ,  3620 ,  3630  and  3640  are combined with one another with the adhesive layer  3601  interposed therebetween (see  FIG. 28B ). 
     Then, if needed, a coat layer (not illustrated) is formed on the peripheral side face of the ceramic block  3603  to complete a honeycomb structure  3600 . 
     The honeycomb structure of the present embodiment is allowed to exert the same effects as those of the honeycomb structure of the first embodiment of the fourth aspect of the present invention. 
     Example 4-3 
     The following description will discuss an example that more specifically discloses the third embodiment of the fourth aspect of the present invention. However, the fourth aspect of the present invention is not limited to the Example. 
     (1) By carrying out the same method as the molding process 
     (1) of Example 1-1, raw honeycomb molded bodies having almost the same shapes as those of the honeycomb fired body  3610  and honeycomb fired body  3640 ′, shown in  FIG. 28A , with no cells sealed, were manufactured. 
     (2) Next, the raw honeycomb molded bodies were dried by using a microwave drying apparatus to obtain a dried honeycomb molded bodies. A paste having the same composition as that of the wet mixture was then filled into predetermined cells, and the filled portions of the dried honeycomb molded bodies were dried by using a drying apparatus again. 
     (3) The dried honeycomb molded bodies were degreased at 400° C., and then fired at 2200° C. under normal pressure argon atmosphere for three hours. 
     Thus, a honeycomb fired body  3610  including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×250 mm, the number of cells (cell density) of 300 pcs/inch 2 , a thickness of cell walls of 0.25 mm (10 mil), and a cross-sectional area of 1190 mm 2  was manufactured. 
     Also, a honeycomb fired body  3640 ′ having the same porosity, the same average pore diameter, the same number of cells (cell density) and the same thickness of cell walls as those of the honeycomb fired body  3610  and also having a trapezoidal cross-sectional shape (upper parallel side=35.5 mm, lower parallel side=70.0 mm, height=34.5 mm) was manufactured. 
     (4) An adhesive paste was applied to a predetermined side face of each of the honeycomb fired bodies  3610  and  3640 ′, and 33 pieces of the honeycomb fired bodies  3610  and eight pieces of the honeycomb fired bodies  3640 ′ were bonded to one another with the adhesive paste interposed therebetween so as to be arranged as shown in  FIG. 28A . The adhesive paste was solidified at 180° C. in 20 minutes to manufacture an aggregated body of the honeycomb fired bodies  3603 ′. 
     Next, the periphery of the aggregated body of the honeycomb fired bodies  3603 ′ was cut by using a diamond cutter to manufacture an almost round pillar-shaped ceramic block  3603  having the cross-sectional area of 49400 mm 2 . 
     With respect to the adhesive paste, the adhesive paste used in Example 1-1 was used. 
     (5) By using a coating material paste having the same composition as that of the adhesive paste used in the process (4), a coating material paste layer was formed on the periphery of the ceramic block  3603 . Thereafter, the coating material paste layer was dried at 120° C. to manufacture a round pillar-shaped honeycomb structure having a size of 254 mm in diameter×250 mm in length with a coat layer formed on the periphery thereof. 
     The cross-sectional shape of the honeycomb structure manufactured in Example 4-3 is shown in  FIG. 27 . 
     The cross-sectional area of the honeycomb fired body  3310  is 1190 mm 2 , the cross-sectional area of the honeycomb fired body  3320  is 1190 mm 2 , the cross-sectional area of the honeycomb fired body  3330  is 1066 mm 2 , the cross-sectional area of the honeycomb fired body  3340  is 1093 mm 2 , the cross-sectional area of the ceramic block  3303  is 49400 mm 2 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3320  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  in the cross section is three, the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3330  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  in the cross section is four, and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  3310  and  3340  and extends from the center of gravity  3303 A of the ceramic block  3303  to the periphery of the ceramic block  3303  in the cross section is four. 
     Comparative Example 4-3 
     (1) By carrying out the same process as the process (1) of Example 4-2, a honeycomb fired body including a silicon carbide sintered body and having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.5 mm×34.5 mm×250 mm, the number of cells (cell density) of 300 pcs/inch 2 , a thickness of cell walls of 0.25 mm (10 mil), and a cross-sectional area of 1190 mm 2  was manufactured. 
     (2) An adhesive paste was applied to a predetermined side face of the honeycomb fired body, and 52 pieces of the honeycomb fired bodies were bonded to one another with the adhesive paste interposed therebetween. The adhesive paste was solidified at 180° C. in 20 minutes to manufacture an aggregated body of the honeycomb fired bodies having a rectangular pillar-shape, with the thickness of the adhesive layer being 1 mm. 
     Here, as the adhesive paste, the same adhesive paste as that used in Example 1-1 was used. 
     (3) Next, the periphery of the aggregated body of the honeycomb fired bodies was cut by using a diamond cutter to manufacture a round pillar-shaped ceramic block having a cross-sectional area of 50511 mm 2 . 
     Subsequently, a coating material paste layer was formed on the periphery of the ceramic block by using the coating material paste made of the same material as that of the adhesive paste. 
     Further, this coating material paste layer was dried at a temperature of 120° C. to manufacture a round pillar-shaped honeycomb structure having a size of 254.2 mm in diameter×250 mm in length. 
     The cross-sectional shape of the honeycomb structure manufactured in Comparative Example 4-3 is shown in  FIG. 29 . 
       FIG. 29  is a cross-sectional view that shows the honeycomb structure  5400  manufactured in Comparative Example 4-3, and in  FIG. 29 , reference numerals  5410 ,  5420 ,  5430 , and  5440  represent honeycomb fired bodies, a reference numeral  5401  represents an adhesive layer, a reference numeral  5402  represents a coat layer and a reference numeral  5403  represents a ceramic block. 
     The cross-sectional area of the honeycomb fired body  5410  is 1190 mm 2 , the cross-sectional area of the ceramic block  5403  is 50511 mm 2 , the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  5410  and  5420  and extends from the center of gravity  5403 A of the ceramic block  5403  to the periphery of the ceramic block  5403  in the cross section is four, the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  5410  and  5430  and extends from the center of gravity  5403 A of the ceramic block  5403  to the periphery of the ceramic block  5403  in the cross section is five, and the number of the adhesive layers existing on a route which passes through the honeycomb fired bodies  5410  and  5440  and extends from the center of gravity  5403 A of the ceramic block  5403  to the periphery of the ceramic block  5403  in the cross section is five. 
     Evaluated in the same manner as in Example 1-1 except that an 8 L engine was used instead of the 2 L engine, the regenerating rate of the honeycomb structure of Example 4-3 was 85%. Further, the regenerating rate of the honeycomb structure of Comparative Example 4-3 was 72%. 
     Other Embodiments of Fourth Aspect of the Present Invention 
     The honeycomb structure according to each of the first and second embodiments of the fourth aspect of the present invention may be manufactured in the same manner as in, for example, the third embodiment of the first aspect of the present invention. 
     The cross-sectional shape of the honeycomb structure according to the embodiments of the fourth aspect of the present invention is not limited to a substantially round shape. The cross-sectional shape may be a substantially elliptical shape, a substantially elongated round shape, a substantially racetrack shape, or the like. 
     Other Embodiments of First to Fourth Aspects of the Present Invention 
     As mentioned above, the honeycomb structure with either one end of each of the cells sealed was described as the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention; however, in the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention, each of the cells is not necessarily sealed at either one end. The honeycomb structure of this kind can be used as a catalyst supporting carrier. 
     The shape of each of the honeycomb fired bodies of the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention is not particularly limited. The shape is preferably a shape which makes it easy to combine the honeycomb fired bodies with one another with the adhesive layer interposed therebetween to manufacture a honeycomb structure, and examples of the cross-sectional shape thereof. 
     Include a substantially square shape, a substantially rectangular shape, a hexagonal shape, a sector shape, and the like. 
     In the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention, examples of the inorganic binder contained in the adhesive paste include silica sol, alumina sol, and the like. Each of these may be used alone, or two or more of these may be used in combination. Out of the inorganic binders, silica sol is preferably used. 
     Examples of the inorganic particles contained in the adhesive paste include carbides, nitrides, and the like, and more specifically, inorganic powder made from silicon carbide, silicon nitride, boron nitride and the like. Each of these may be used alone, or two or more kinds of these may be used in combination. Out of the inorganic particles, silicon carbide is preferably used due to its superior thermal conductivity. 
     Examples of at least one of the inorganic fibers and whiskers contained in the adhesive paste include at least one of inorganic fibers and whiskers made of silica-alumina, mullite, alumina, silica or the like. Each of these materials may be used alone, or two or more of these may be used in combination. Out of the inorganic fibers, alumina fibers are preferably used. 
     Although not particularly limited, a porosity of the honeycomb fired body of the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention is preferably at least about 35% and at most about 60%. 
     When the honeycomb structure is used as a filter, the porosity of about 35% or more is less likely to cause clogging in the honeycomb structure. In contrast, the porosity of about 60% or less is less likely to cause a reduction in the strength of the honeycomb fired body, so that the honeycomb fired body is less likely to be easily broken. 
     The average pore diameter of the honeycomb fired body of the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention is preferably at least about 5 μm and at most about 30 μm. 
     When the honeycomb structure is used as a filter, the average pore diameter of about 5 μm or more is less likely to easily cause clogging of particulates. In contrast, the average pore diameter of about 30 μm or less is less likely to allow particulates to pass through the pores. As a result, the honeycomb fired body may easily capture the particulates, and thus, the honeycomb structure may function as a filter for sure. 
     Here, the porosity and the average pore diameter can be measured by conventionally known methods such as a mercury porosimetry, Archimedes method, and a measuring method using a scanning electronic microscope (SEM). 
     The cell density in the cross-section perpendicular to the longitudinal direction of the honeycomb fired body constituting the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention is not particularly limited. However, a preferable lower limit thereof is about 31.0 pcs/cm 2  (about 200 pcs/inch  2 ) and a preferable upper limit is about 93.0 pcs/cm 2  (about 600 pcs/inch 2 ). A more preferable lower limit is about 38.8 pcs/cm 2  (about 250 pcs/inch 2 ) and a more preferable upper limit is about 77.5 pcs/cm 2  (about 500 pcs/inch 2 ). 
     Further, the thickness of the cell walls of the honeycomb fired body constituting the honeycomb structure is not particularly limited, and preferably at least about 0.1 mm and at most about 0.4 mm. 
     The main component of the honeycomb fired body constituting the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention is not limited to silicon carbide, and may be powders of the following ceramics: nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; carbide ceramics such as zirconium carbide, titanium carbide, tantalum carbide, and tungsten carbide; oxide ceramics such as cordierite, aluminum titanate; and the like. 
     Out of these components, non-oxide ceramics are preferable, and silicon carbide is particularly preferable. This is because they are excellent in thermal resistance, mechanical strength, thermal conductivity and the like. Moreover, ceramic materials such as silicon-containing ceramics, in which the above-mentioned ceramic is blended with metallic silicon, and ceramics bonded by silicon or silicate compounds can also be used as the constitutional material. Out of these, silicon carbide blended with metallic silicon (silicon-containing silicon carbide) is preferable. 
     In particular, ceramics of silicon-containing silicon carbide including about 60% by weight or more of silicon carbide are preferable. 
     The particle diameter of the ceramic powder is not particularly limited, and the silicon carbide powder that tends not to cause the case where the size of the honeycomb fired body manufactured by the following firing treatment becomes smaller than that of the honeycomb molded body after degreased is preferable. 
     With respect to the wet mixture prepared upon manufacturing the honeycomb structure according to each of the embodiments of the first to fourth aspects of the present invention, the organic binder to be mixed in the wet mixture is not particularly limited, and examples thereof include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, and the like. Methyl cellulose is preferable out of these. The binder is preferably blended at a ratio of at least about 1 part by weight and at most about 10 parts by weight per 100 parts by weight of the ceramic powder. 
     The plasticizer to be mixed in the wet mixture is not particularly limited, and examples thereof include glycerin and the like. 
     Also, the lubricant to be mixed in the wet mixture is not particularly limited, and examples thereof include polyoxyalkylene compounds such as polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, and the like. Specific examples of the lubricant include polyoxyethylene monobutyl ether, polyoxypropylene monobutyl ether, and the like. 
     Also, in some cases, the plasticizer or lubricant may not be mixed in the wet mixture. 
     Also, when preparing the wet mixture, it is acceptable to use a dispersant solution such as water, organic solvents such as benzene, and alcohol such as methanol. 
     Further, it is also acceptable to add a forming auxiliary to the wet mixture. 
     The forming auxiliary is not particularly limited, and examples thereof include ethylene glycol, dextrin, fatty acids, fatty acid soap, polyalcohol, and the like. 
     Further, it is acceptable to add balloons, which are fine hollow spheres containing oxide ceramic as a component, and a pore-forming agent such as spherical acrylic particles or graphite to the wet mixture, if necessary. 
     The balloons are not particularly limited, and examples thereof include alumina balloons, glass micro balloons, shirasu balloons, fly ash balloons (FA balloons), mullite balloons, and the like. Alumina balloons are preferable out of these. 
     The plug material paste for sealing the cells is not particularly limited, and the plug, manufactured in the subsequent process, preferably has a porosity of at least about 30% and at most about 75%. For example, it is possible to use a paste-like material, which is the same material as the wet mixture. 
     A catalyst for converting exhaust gases may be supported on the honeycomb structure according to the embodiments of the first to fourth aspects of the present invention, and the catalyst to be supported is desirably noble metals such as platinum, palladium, and rhodium. Out of these, platinum is more desirably used. Moreover, alkali metals such as potassium and sodium, or alkali-earth metals such as barium may be used as other catalysts. Each of these catalysts may be used alone, or two or more kinds of these may be used in combination. 
     In the combining process in the method for manufacturing the honeycomb structure of each of the embodiments of the first to fourth aspects of the present invention, instead of the method in which an adhesive paste is applied to a side face of each honeycomb fired body, for example, a method may be used in which, with each of honeycomb fired bodies being temporarily secured in a mold frame having almost the same shape as the shape of a ceramic block (or an aggregated body of honeycomb fired bodies) to be manufactured, an adhesive paste is injected between each of the honeycomb fired bodies. 
     Each of the honeycomb structure according to the embodiments of the first to fourth aspects of the present invention may also have the characteristics of other aspects of the present invention. 
     For example, in addition to the above characteristic, the honeycomb structure according to the embodiments of the first aspect of the present invention may have the following characteristics, that is: provided that a figure, which is similar to the shape of the ceramic block in the cross section and is concentric with the shape of the ceramic block in the cross section, is drawn in the cross section with an area ratio of the figure being about 49% to the area of the ceramic block in the cross section, a part of each of the peripheral-portion honeycomb fired bodies is necessarily located in the figure; the honeycomb structure includes the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer, and at least one of the first peripheral-portion adhesive layers and the second peripheral-portion adhesive layer form an angle of at least about 40° and at most about 50°; or the cross-sectional area of the ceramic block and the number of the adhesive layers existing on a route which extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section satisfy the predetermined relationships. 
     Further, for example, in addition to the above characteristic, the honeycomb structure according to the embodiments of the second aspect of the present invention may have the following characteristics, that is: the honeycomb structure includes the first peripheral-portion adhesive layer and the second peripheral-portion adhesive layer, and at least one of the first peripheral-portion adhesive layers and the second peripheral-portion adhesive layer forms an angle of at least about 40° and at most about 50°; or the cross-sectional area of the ceramic block and the number of the adhesive layers existing on a route which extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section satisfy the predetermined relationships. 
     Further, for example, in addition to the above characteristic, the honeycomb structure according to the embodiments of the third aspect of the present invention may have the following characteristic, that is: the cross-sectional area of the ceramic block and the number of the adhesive layers existing on a route which extends from the center of gravity of the ceramic block to the periphery of the ceramic block in the cross section satisfy the predetermined relationships. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.