Recently, particulates such as soot, that are contained in exhaust gases discharged from internal combustion engines of vehicles, such as buses, trucks and the like, and construction machines and the like, have raised serious problems as those particulates are harmful to the environment and the human body.  Conventionally, there have been proposed various filters used for collecting particulates in exhaust gases to purify the exhaust gases, and there have also been proposed filters having a honeycomb structure. 
FIG.  4 is a perspective view that shows one type of filter having a honeycomb structure of this type. 
This honeycomb filter 60, which is configured as a honeycomb structural body made of silicon carbide and the like, has a structure in that a plurality of square-pillar shaped porous ceramic members 70 are combined with one another through a sealing material layer 64 that serves as an adhesive to configure a ceramic block 65, and a sealing material layer 63 is also formed on the circumference of this ceramic block 65. 
FIG.  5(a) is a perspective view that schematically shows the porous ceramic member constituting the honeycomb filter shown in FIG.  4, and FIG.  5(b) is a cross-sectional view taken along line B-B of the porous ceramic member shown in FIG.  5(a). 
The porous ceramic member 70 has a honeycomb structure in which a partition wall 73, which separates a large number of through holes 71 placed in parallel with one another in the length direction from one another, functions as a filter. 
In other words, as shown in FIG.  5(b), each of the through holes 71, formed in the porous ceramic member 70, is sealed with a sealing member 72 at one of ends on its exhaust gas inlet side or exhaust gas outlet side, so that exhaust gases that have entered one through hole 71 are discharged from another through hole 71 after having necessarily passed through the partition wall 73 that separates the through holes 71 from one another. 
Here, the sealing material layer 63 formed on the circumference is provided for the purpose of preventing exhaust gases from leaking from the peripheral portion of the ceramic block 65 when the honeycomb filter 60 is installed in an exhaust passage of an internal combustion engine. 
When the honeycomb filter 60 having such a structure is installed in the exhaust passage of the internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 73 upon passing through the honeycomb filter 60, so that the exhaust gases are purified. 
With respect to the filters having the honeycomb structure, in addition to the structure that a plurality of porous ceramic members are combined with one another, those filters formed as a single integral ceramic body made of cordierite or the like as a whole, those honeycomb filters formed through an extrusion-molding process by using inorganic fibers made of alumina, silica, mullite or the like, and those honey comb filters that are formed by subjecting an inorganic sheet, made of inorganic fibers through a paper-making process, or a metal sheet to a corrugating process into a roll shape have been known (for example, see Patent Documents 1, 2 and 3). 
The honeycomb filters having the above-mentioned structures are superior in heat resistance, and particulate burning and removing processes (hereinafter, referred to as a regenerating process), and the like are easily carried out thereon; therefore, these honeycomb filters are used for various large-size vehicles, diesel-engine-installed vehicles and the like. 
Moreover, there have been known filters, which collect particulates in exhaust gases and are also capable of purifying toxic exhaust gases such as CO, HC, NOx and the like.  In these filters, a catalyst used for purifying exhaust gases is adhered to a portion (through holes and the like) functioning as a filter. 
In the honeycomb structural body to which the catalyst is adhered, since particulates are deposited on the catalyst, active energy, required for burning particulates, is reduced by the catalyst, so that particulates can be burned even at low temperatures.  Therefore, conventionally, attempts have been made to burn particulates at low temperatures and improve the purifying performance for exhaust gases by improving dispersibility of the catalyst to increase reaction sites. 
In the above-mentioned filter using a catalyst, the regenerating and purifying processes are carried out by using the following two kinds of methods. 
In a first method, although the purifying process for toxic gases in exhaust gases is continuously carried out, the regenerating process is not started until the collected particulates have reached a certain amount of deposition.  After having reached the amount of deposition, the regenerating process is carried out to remove the particulates, and particulates are again collected.  Thus, these processes are repeated several times. 
In a second method, while the purifying process for toxic gases in exhaust gases is continuously carried out, the burning and removing processes of particulates are also carried out continuously, so that the particulates are successively burned without being deposited. 
In order to exert efficient reactions at a low pressure loss by using these methods, it is preferable to increase reaction sites between particulates and the catalyst; therefore, it is considered to be preferable that the specific surface area of the honeycomb structural body be increased. 
In this case, however, when a method is adopted in which the specific surface area of the honeycomb structural body is expanded by increasing the number of through holes per unit cross sectional area, with the cross sectional area of the through holes of the honeycomb structural body being made smaller, the small cross-sectional area of the through hole makes it difficult for exhaust gases to flow through it to cause a high pressure loss, failing to provide a practical method. 
Another effective method has been proposed in which the density of the wall portion constituting the filter is lowered to increase the porosity, so that a large number of open pores are included therein; thus, even pore portions in deeper layers of the filter wall portion are allowed to collect particulates so as to make the particulates in contact with the catalyst located inside the wall portion. 
However, when the above-mentioned methods are used in the above-mentioned filters, the strength of the filer becomes lower.  In particular, in the case of the filter disclosed in Patent Document 1, the strength of the filter becomes considerably low.  For this reason, upon burning and removing collected particulates (hereinafter, referred to as a regenerating process), the filter of this type is likely to be suffered from a great temperature difference in the length direction of the filter accompanied with the burning process of the particulates, resulting in damages such as cracks in the filter due to the subsequent thermal stress.  Consequently, the above-mentioned filters tend to lose functions as the filter. 
Moreover, in order to effectively utilize exhaust heat generated from the engine so as to carry out regenerating and purifying processes, the filter is desirably installed immediately under the engine; however, the installation space is extremely limited.  For this reason, the filter needs to be formed into a complex shape; however, it is very difficult to form the conventional filter into a complex shape. 
Furthermore, laminated filters have also been proposed; however, these filters are strengthened in toughness by adding ceramic fibers to a ceramic material (clay), lacking in the idea of realizing high porosity (see Patent Document 4). 
Patent Document 1: JP-A 06-182228 (1994)
Patent Document 2: JP-A 04-2674 (1992)
Patent Document 3: JP-A 2001-252529
Patent Document 4: JP-A 08-12460 (1996)