Patent Publication Number: US-2009237080-A1

Title: Magnetic field generator and nuclear magnetic resonance device provided with the magnetic field generator

Description:
TECHNICAL FIELD 
     The present invention relates to a magnetic field generator and a nuclear magnetic resonance device provided with the magnetic field generator, and more particularly to a magnetic field generator having a magnetic circuit of an open magnetic path in which a plurality of magnetic field generating units is arranged laterally without adopting arrangement in which a plurality of magnetic field generating unit is arranged to face each other. 
     BACKGROUND ART 
     Conventionally, in a nuclear magnetic resonance device, a region having uniform and stable magnetic flux density is formed using a counter-type magnetic field generator which arranges a permanent magnet of N pole and a permanent magnet of S pole in an opposedly-facing manner with a person who is a subject sandwiched therebetween, and a predetermined inspection is performed. 
     Accordingly, the magnetic field generator is required to form a relatively large gap which allows the insertion of the subject between the opposedly facing permanent magnets. Due to such constitution, the permanent magnets are required to generate a larger magnetic force and hence, the use of larger-sized permanent magnets becomes necessary thus making the magnetic field generator large-sized and pushing up a manufacturing cost. 
     Such a nuclear magnetic resonance device is extremely effective in observing the inside of a body of the subject. However, for example, in the observation of a part of the subject which is relatively close to a surface of a body such as an eye, an ear, a nose or a tooth, for example, ability of the nuclear magnetic resonance device exceeds ability necessary for the observation of such a part. 
     Accordingly, in place of the counter-type magnetic field generator which constitutes a magnetic circuit of a closed magnetic path which arranges the N pole and the S pole in an opposedly facing manner, there has been proposed a magnetic field generator which constitutes a magnetic circuit of an open magnetic path in which an N pole and an S pole are arranged in the lateral direction (see patent document 1, for example). 
     In such a magnetic field generator, respective one-magnetic-pole sides of two permanent magnets having a rectangular parallelepiped shape are connected to a yoke having a rectangular parallelepiped shape thus forming a so-called U-shaped magnet having a U shape, and an N pole and an S pole are formed on distal ends of the U-shaped magnet thus forming a magnetic circuit of an open magnetic path. 
     Further, an auxiliary magnet is arranged between an end portion of U-shaped magnet which forms the N pole and the end portion of the U-shaped magnet which forms the S pole thus forming a region where magnetic flux density is set uniform due to an interaction between the auxiliary magnet and the U-shaped magnet. 
     Further, as another magnetic field generator, there has been proposed a technique in which, without using a U-shaped magnet, a plurality of magnets which respectively has predetermined lengths and directs the respective directions of magnetization in predetermined directions is arranged laterally thus forming a region where the magnetic flux density is set uniform (see non-patent document 1, for example). 
     [Patent Document 1] JP-A-2003-250777 
     [Non-Patent document 1] The Fundamental Investigation of Optimal Design of Magnetic Circuit for Open Type MRI equipment by Yoshinori Okamoto and others, Denki Gakkai (Institute of Electrical Engineers of Japan), Study Group on Material of Magnetics MAG-04-156, 2004 
     DISCLOSURE OF THE INVENTION 
     Tasks to be Solved by the Invention 
     However, with respect to the magnetic field generator disclosed in JP-A-2003-250777 which constitutes the magnetic circuit of an open-magnetic path using the U-shaped magnet and the auxiliary magnet, a range where a region having the uniform magnetic flux density is formed is small in size and hence, it is difficult to acquire a region having a desired size. 
     Further, in the magnetic field generator which constitutes the magnetic circuit of an open-magnetic path using the plurality of magnets which makes the respective directions of magnetization different from each other and, at the same time, makes the respective lengths thereof different from each other, the number of kinds of required magnets becomes extremely large and hence, the magnetic field generator is easily influenced by irregularities of performances of the magnets per se thus increasing irregularities of the region having the uniform magnetic flux density generated for every magnetic field generator whereby there arises a drawback that an adjustment operation for suppressing the irregularities of the magnetic flux density is extremely difficult. 
     Under such circumstances, inventors of the present invention have made researches and developments for providing a magnetic field generator which is constituted of a magnetic circuit of an open-magnetic path which can form a uniform magnetic field having a simpler structure, and have arrived at the present invention. 
     The present invention is directed to a magnetic field generator of an open magnetic path type which is configured to form a region having magnetic flux density of a uniform magnitude at a predetermined position away from a surface of a casing using a plurality of permanent magnets housed in the casing, the magnetic field generator including a first permanent magnet and a second permanent magnet which direct directions of magnetization in a reference direction opposite to a direction of the magnetic flux density in the region, and are sequentially arranged in the reference direction with a predetermined distance therebetween; and a third permanent magnet and a fourth permanent magnet which are sequentially arranged between the first permanent magnet and the second permanent magnet in a state that the third permanent magnet and the fourth permanent magnet are brought into contact with each other or are arranged with a predetermined distance therebetween along the reference direction, wherein a direction of magnetization of the third permanent magnet is set to a direction having a first component which is directed parallel to the reference direction and a second component which is directed toward the region side orthogonal to the first direction, and a direction of magnetization of the fourth permanent magnet is set to a direction having the first component and an inverted second component having a magnitude equal to a magnitude of the second component in a direction opposite to a direction of the second component. 
     Further, the magnetic field generator of the present invention is also characterized by following technical features. 
     (1) The third permanent magnet and the fourth permanent magnet are respectively formed of a permanent magnet which includes a non-magnetic-material region where no magnetic material is present, and is configured to adjust apparent residual magnetic flux density by adjusting a size of the non-magnetic-material region and, thereafter, by magnetizing the magnetic material with saturated residual magnetic flux density by an external magnetic field having a predetermined intensity. 
     (2) The adjustment of size of the non-magnetic region is the adjustment of at least any one of size, number and depth of holes or slits formed in the permanent magnet. 
     (3) The adjustment of size of the non-magnetic region is the adjustment of a mixing ratio of a plurality of lump permanent magnets having a sufficiently small size compared to a volume of the third permanent magnet and a volume of the fourth permanent magnet and a non-magnetic material in a permanent magnet which is formed by mixing the plurality of permanent magnets and the non-magnetic material and by forming the lump permanent magnets and the non-magnetic material into an integral body having a predetermined shape. 
     (4) The adjustment of size of the non-magnetic region is the adjustment of a content ratio of a plurality of permanent magnet blocks and a plurality of non-magnetic-material blocks having the same shape as the permanent magnet blocks in a permanent magnet which is formed by integrally bonding the permanent magnet blocks and the non-magnetic-material blocks. 
     (5) The first permanent magnet and the second permanent magnet are integrally formed by one cylindrical permanent magnet having a cylindrical shape, two pieces of third permanent magnets and two pieces of fourth permanent magnets are respectively arranged in a hollow center portion of the cylindrical permanent magnet in such a state that said two pieces of the third permanent magnets and said two pieces of said fourth permanent magnets are respectively arranged in a mirror symmetry with respect to a symmetry plane which passes the center of the cylindrical permanent magnet and is parallel to the reference direction, a first inner permanent magnet is mounted on a symmetry surface side of the third permanent magnets respectively, and a first outer permanent magnet is mounted on a side opposite to the first inner permanent magnet with the third permanent magnet sandwiched therebetween respectively, a second inner permanent magnet is mounted on a symmetry surface side of the fourth permanent magnets respectively, and a second outer permanent magnet is mounted on a side opposite to the second inner permanent magnet with the fourth permanent magnet sandwiched therebetween respectively, and a direction of magnetization of the first inner permanent magnet is directed in the direction of the second component, a direction of magnetization of the first outer permanent magnet is directed in a direction opposite to the direction of the second component, a direction of magnetization of the second inner permanent magnet is directed in a direction opposite to the direction of the second component, and a direction of magnetization of the second outer permanent magnet is directed in a direction of the second component. 
     (6) A fifth permanent magnet is arranged between the first permanent magnet and the third permanent magnet, and a direction of magnetization of the fifth permanent magnet is directed in a combined direction of a third component parallel to or antiparallel to the reference direction and a fourth component which is directed toward a region side orthogonal to the reference direction, and a sixth permanent magnet is arranged between the second permanent magnet and the fourth permanent magnet, and a direction of magnetization of the sixth permanent magnet is directed in a combined direction of the third component and an inverted fourth component which is directed in a direction opposite to a direction of the fourth component and has the same magnitude as the fourth component. 
     (7) A fifth permanent magnet which has a direction of magnetization thereof directed in the direction parallel to the second component is arranged between the first permanent magnet and the third permanent magnet, and a sixth permanent magnet which has a direction of magnetization thereof directed in the direction parallel to the inverted second component is arranged between the second permanent magnet and the fourth permanent magnet. 
     (8) The first permanent magnet and the second permanent magnet are integrally formed by one cylindrical permanent magnet having a cylindrical shape, and 
     the third permanent magnet and the fourth permanent magnet are respectively arranged in a hollow center portion of the cylindrical permanent magnet. 
     (9) The first permanent magnet is constituted of two permanent magnets which are arranged in a mirror symmetry with respect to a symmetry plane parallel to the reference direction, and said two permanent magnets respectively include a magnetization component which is directed in a direction orthogonal to the symmetry plane and toward the symmetry plane, and the second permanent magnet is constituted of two permanent magnets which are arranged in a mirror symmetry with respect to the symmetry plane, and said two permanent magnets respectively include a magnetization component which is directed in a direction orthogonal to the symmetry plane and in a direction opposite to the symmetry plane. 
     (10) The third permanent magnet is constituted of two permanent magnets which are arranged in a mirror symmetry with respect to a symmetry plane parallel to the reference direction, and said two permanent magnets respectively include a magnetization component which is directed in a direction orthogonal to the symmetry plane and toward the symmetry plane, and the fourth permanent magnet is constituted of two permanent magnets which are arranged in a mirror symmetry with respect to the symmetry plane, and said two permanent magnets respectively include a magnetization component which is directed in a direction orthogonal to the symmetry plane and in a direction opposite to the symmetry plane. 
     Further, a nuclear magnetic resonance device of the present invention is characterized in that the nuclear magnetic resonance device is provided with the above-mentioned magnetic field generator. 
     Advantage of the Invention 
     According to the magnetic field generator of the present invention, in the magnetic field generator which is configured to form a region having magnetic flux density of a uniform magnitude at a predetermined position away from a surface of a casing using a plurality of permanent magnets housed in the casing, the magnetic field generator includes: a first permanent magnet and a second permanent magnet which direct directions of magnetization in a reference direction opposite to a direction of the magnetic flux density in the region, and are sequentially arranged in the reference direction with a predetermined distance therebetween; and a third permanent magnet and a fourth permanent magnet which are sequentially arranged between the first permanent magnet and the second permanent magnet in a state that the third permanent magnet and the fourth permanent magnet are brought into contact with each other or are arranged with a predetermined distance therebetween along the reference direction, a direction of magnetization of the third permanent magnet is set to a direction having a first component which is directed parallel to the reference direction and a second component which is directed toward the region side orthogonal to the first direction, and a direction of magnetization of the fourth permanent magnet is set to a direction having the first component and an inverted second component having a magnitude equal to a magnitude of the second component in a direction opposite to a direction of the second component. Due to such constitution, it is possible to provide the magnetic field generator constituted of a magnetic circuit of an open-magnetic path which can generate a uniform magnetic field by four permanent magnets. 
     Further, the third permanent magnet and the fourth permanent magnet are respectively formed of a permanent magnet which includes a non-magnetic-material region where no magnetic material is present, and is configured to adjust apparent residual magnetic flux density by adjusting a size of the non-magnetic-material region and, thereafter, by magnetizing the magnetic material with saturated residual magnetic flux density by an external magnetic field having a predetermined intensity. Due to such constitution, residual magnetic flux densities of the third permanent magnet and the fourth permanent magnet are set to desired residual magnetic flux densities with extremely high accuracy and hence, adjustment accuracy of the magnetic field by the third permanent magnet and the fourth permanent magnet can be enhanced thus increasing the region having the uniform magnetic field. 
     Further, by arranging the fifth permanent magnet between the first permanent magnet and the third permanent magnet, and by arranging the sixth permanent magnet between the second permanent magnet and the fourth permanent magnet, it is possible to increase the region having the uniform magnetic field. 
     Further, the nuclear magnetic resonance device of the present invention uses the magnetic field generator which includes at least four permanent magnets. Due to such constitution, the magnetic field generator can be light-weighted and manufactured at a low cost and, at the same time, it is possible to provide a nuclear magnetic resonance device which can be easily installed and maintained at a low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a schematic longitudinal cross-sectional view of a magnetic field generator of a first embodiment; 
         FIG. 2  is a view showing magnetic flux density distribution of a magnetic field generated by the magnetic field generator of the first embodiment; 
         FIG. 3  is a graph showing a state of magnetic flux density along an Z axis at X=0 in  FIG. 2 ; 
         FIG. 4  is a graph showing a state of magnetic flux density along an X axis in  FIG. 2 ; 
         FIG. 5  is a graph showing a state of magnetic flux density along the X axis at X=0; 
         FIG. 6  is an explanatory view of a state of arrangement of respective permanent magnets which becomes a basis of calculation performed in  FIG. 5 ; 
         FIG. 7  is a graph showing dependency of magnitude of magnetization of a fourth permanent magnet with respect to uniformity of a target region 
         FIG. 8  is a schematic longitudinal cross-sectional view of a magnetic field generator of a modification of the first embodiment; 
         FIG. 9  is an end surface view as viewed from a line Y 1 -Y 1  in  FIG. 8 ; 
         FIG. 10  is an end surface view as viewed from a line Y 2 -Y 2  in  FIG. 8 ; 
         FIG. 11  is a schematic longitudinal cross-sectional view of a magnetic field generator of a second embodiment; 
         FIG. 12  is a view showing magnetic flux density distribution of a magnetic field generated by the magnetic field generator of the second embodiment; 
         FIG. 13  is a graph showing a state of magnetic flux density along a Z axis at X=0 in  FIG. 12 ; 
         FIG. 14  is a graph showing a state of magnetic flux density along an X axis in  FIG. 12 ; 
         FIG. 15  is an explanatory view of a state of arrangement of first to sixth permanent magnets in the magnetic field generator of the second embodiment; 
         FIG. 16  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; 
         FIG. 17  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; 
         FIG. 18  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; 
         FIG. 19  is an explanatory view of a state of magnetization of the third permanent magnet and the fourth permanent magnet of a modification; 
         FIG. 20  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; 
         FIG. 21  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; 
         FIG. 22  is an explanatory view of a modification of a state of arrangement of first to sixth permanent magnets; and 
         FIG. 23  is a schematic longitudinal cross-sectional view of a magnetic field generator for a nuclear magnetic resonance device provided with a reception coil and a transmission coil. 
     
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS 
     
         
         A 1 : magnetic field generator 
         A 2 : magnetic field generator 
           10 : casing 
           10   a : main surface 
           11 : first permanent magnet 
           12 : second permanent magnet 
           13 : third permanent magnet 
           14 : fourth permanent magnet 
           15 : fifth permanent magnet 
           16 : sixth permanent magnet 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A magnetic field generator and a nuclear magnetic resonance device which uses such a magnetic field generator according to the present invention are characterized by the use of a magnetic field generator which is constituted of a magnetic circuit of an open-magnetic path. Particularly, by forming such a magnetic field generator using at least four permanent magnets, the magnetic field generator can be extremely miniaturized and light-weighted. 
     Embodiments of the present invention are explained in detail in conjunction with drawings hereinafter.  FIG. 1  is a schematic longitudinal cross-sectional view of a magnetic field generator A 1  of the first embodiment. The magnetic field generator A 1  is configured such that a first permanent magnet  11  and a second permanent magnet  12  are arranged in one direction with a predetermined distance therebetween in the inside of a casing  10  having a planar main surface  10   a  on one side surface thereof, and a third permanent magnet  13  and a fourth permanent magnet  14  are arranged between the first permanent magnet  11  and the second permanent magnet  12 . Particularly, the third permanent magnet  13  is arranged close to the first permanent magnet  11 , and the fourth permanent magnet  14  is arranged close to the second permanent magnet  12 . Here, these permanent magnets  11  to  14  are arranged in order of the first permanent magnet  11 , the third permanent magnet  13 , the fourth permanent magnet  14  and the second permanent magnet  12 , and this direction of arrangement of the first to fourth permanent magnets  11  to  14  is set as a reference direction. 
     Using such first to fourth permanent magnets  11  to  14 , the magnetic field generator A 1  can form a region having magnetic flux density of uniform magnitude at a position spaced apart from the main surface  10   a  by a predetermined distance as shown in  FIG. 1 . The region having magnetic flux density of uniform magnitude is referred to as a target region T. In this target region T, the direction of magnetic flux density is parallel to the main surface  10   a  and is directed opposite to the reference direction. 
     In this embodiment, the first to fourth permanent magnets  11  to  14  are formed of rectangular parallelepiped bodies having predetermined sizes respectively, and are arranged such that respective one-end surfaces of the first to fourth permanent magnets  11  to  14  are brought into contact with a surface of the casing  10  opposite to the main surface  10   a.    
     Although not shown in the drawing, plate-shaped ribs are provided in the inside of the casing  10  at predetermined positions for reinforcing the casing  10 , and the first to fourth permanent magnets  11  to  14  are held at predetermined positions by adjusting the arrangement of these ribs. 
     A magnitude of magnetization of the first permanent magnet  11  and a magnitude of magnetization of the second, permanent magnet  12  are set equal to each other as described later. Further, a magnitude of magnetization of the third permanent magnet  13  and a magnitude of magnetization of the fourth permanent magnet  14  are also set equal to each other as described later. 
     The direction of magnetization of the first permanent magnet  11  and the direction of magnetization of the second permanent magnet  12  are respectively set parallel to the reference direction as indicated by arrows in  FIG. 1 . 
     Further, the direction of magnetization of the third permanent magnet  13  is, as indicated by an arrow in  FIG. 1 , directed in the direction which makes a predetermined angle α with respect to the reference direction and, particularly, the third permanent magnet  13  has the direction of magnetization thereof directed toward a main surface  10   a  side. 
     Further, the direction of magnetization of the fourth permanent magnet  14  is, as indicated by an arrow in  FIG. 1 , directed in the direction which makes a predetermined angle α with respect to the reference direction and, particularly, the fourth permanent magnet  14  has the direction of magnetization thereof directed toward a side opposite to the main surface  10   a  side. 
     That is, assuming the magnitude of magnetization of the third permanent magnet  13  and the magnitude of magnetization of the fourth permanent magnet  14  as m, the third permanent magnet  13  and the fourth permanent magnet  14  respectively have a first component of a magnitude of m cos α in the reference direction. Further, the third permanent magnet  13  has a second component having a magnitude of m sin α in the direction orthogonal to the reference direction and is also directed toward the main surface  10   a  side, and the fourth permanent magnet  14  has an inverted second component having a magnitude of m sin α in the direction orthogonal to the reference direction and is also directed to a side opposite to the main surface  10   a  side. That is, the third permanent magnet  13  and the fourth permanent magnet  14  have magnetization components of the same magnitude in the directions orthogonal to the reference direction and opposite to each other. 
     A magnitude of an angle α is set to a proper value by taking a balance between the magnitude of magnetization of the third permanent magnet  13  and the magnitude of magnetization of the fourth permanent magnet  14 . By setting the magnitude of magnetization of the third permanent magnet  13  and the magnitude of magnetization of the fourth permanent magnet  14  such that the angle α becomes smaller than 45°, it is possible to increase a size of the region having the uniform magnetic flux density which is generated by the magnetic field generator A 1 . In the region having uniform magnetic flux density, the direction of the magnetic flux density is set opposite to the reference direction. 
     The magnetic field generator A 1  having such constitution can form the region having the uniform magnetic flux density at a place which is spaced apart from the main surface  10   a  by a predetermined distance. It is confirmed that a magnetic field having the magnetic flux density distribution shown in  FIG. 2  can be formed based on a numerical value analysis using a model case which is formed with following particulars. As sizes of the first permanent magnet  11  and the second permanent magnet  12 , cross sections shown in  FIG. 1  are set to 10 cm×20 cm. As sizes of the third permanent magnet  13  and the fourth permanent magnet  14 , cross sections shown in  FIG. 1  are set to 5 cm×10 cm. A distance of approximately 5 cm is set between the first permanent magnet  11  and the third permanent magnet  13  as well as between the fourth permanent magnet  14  and the second permanent magnet  12 . Residual magnetic flux density of the first permanent magnet  11  and the second permanent magnet  12  is set to 1.38 T and residual magnetic flux density of the third permanent magnet  13  and the fourth permanent magnet  14  is set to 1.05 T. A magnitude of an angle α is set to 16.5°. 
       FIG. 3  shows a magnitude of magnetic flux density in the Z axis direction when X=0 in  FIG. 2 .  FIG. 4  shows a magnitude of magnetic flux density in the X axis direction from X=0 when Z=5.75 cm, 6.0 cm, 6.25 cm in  FIG. 2 . It is understood from  FIG. 3  and  FIG. 4  that the target region T having uniform magnetic flux density is formed at a position approximately 6 cm from the surface of the casing  10  opposite to the main surface  10   a  which constitutes one end peripheries of the first to forth permanent magnet  11  to  14 . 
     Here, the third permanent magnet  13  and the fourth permanent magnet  14  are arranged in a state that the third permanent magnet  13  and the fourth permanent magnet  14  are brought into contact with each other. However, it is not always necessary to bring these permanent magnets  13 ,  14  into contact with each other, and these permanent magnets  13 ,  14  may be arranged in a spaced-apart manner with a predetermined distance therebetween. When these permanent magnets  13 ,  14  are arranged in a spaced-apart manner, the allowable distance is approximately 3 cm. The distance between the third permanent magnet  13  and the fourth permanent magnet  14  may be adjusted by adjusting magnitudes and directions of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14 . 
     Magnitudes of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14  largely influence a size of the region having uniform magnetic flux density and hence, it is desirable to set magnitudes of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14  to predetermined values with high accuracy as much as possible. 
       FIG. 5  is a graph showing a result of a test which is performed for checking influence on making magnetic flux density uniform due to the fourth permanent magnet  14  or the third permanent magnet  13 . Here, the graph shows a result of calculation of magnitudes of magnetic flux density on the Z axis when Z=0 under conditions shown in  FIG. 6 . That is, in  FIG. 6 , the second permanent magnet  12   p  having a length of 10 cm in the X magnetic axis direction and a length of 20 cm in the Z axis direction is arranged such that one end periphery of the second permanent magnet  12   p  is positioned on the X axis 10 cm away from an origin of X-Z coordinates, and the fourth permanent magnet  14   p  having a length of 6 cm in the X magnetic axis direction and a length of 10 cm in the Z axis direction has one side periphery there of positioned on the Z axis and has one end periphery thereof arranged from the axis X by a distance of 1.5 cm, and the magnitude of magnetization of the fourth permanent magnet  14   p  is set to values relative to the magnitude of magnetization of the second permanent magnet  12   p  which becomes the reference, that is, “1.000”, “0.983”, “0.966”, “0.949”, “0.932”, “0.915”, “0.898”, “0.881”, “0.864”, “0.847” and “0.830”. 
     Here, the direction of magnetization of the second permanent magnet  12   p  is directed in the direction parallel to the reference direction, and the direction of magnetization of the fourth permanent magnet  14   p  is directed in the direction which sets the above-mentioned angle α to 13°. 
     It is apparent from  FIG. 5  that the magnitude of magnetization of the fourth permanent magnet  14   p  influences the size of the region having the uniform magnetic flux density. Further,  FIG. 7  is a graph which compares uniformity with respect to a magnitude of relative residual magnetic flux density of the fourth permanent magnet  14   p . It is understood from the graph that an optimum value exists with respect to the uniformity. It is understood from the graph that, in this embodiment, it is desirable to set the magnitude of magnetization of the fourth permanent magnet  14   p  to approximately 0.915 of the magnitude of magnetization of the second permanent magnet  12   p . Here, uniformity is a value which is acquired by dividing the difference between a maximum value and a minimum value of magnetic flux density in an estimated region with a value at the center of the target region and by multiplying the divided value with 1×106. 
     In this manner, the magnitudes of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14  largely influence the enhancement of uniformity of the magnetic flux density in the target region and hence, in the third permanent magnet  13  and the fourth permanent magnet  14 , a non-magnetic-material region where a magnetic material is not present is formed, a size of the non-magnetic-material region is adjusted and, thereafter, the magnetic material is magnetized to saturated residual magnetic flux density by an external magnetic field having predetermined intensity thus adjusting apparent residual magnetic flux density. 
     In a method of adjusting the size of the non-magnetic region, holes or slits are formed in a magnetic-material having a predetermined shape which is not yet magnetized thus forming pores or air gaps which form the non-magnetic regions while maintaining a profile shape of the magnetic-material in a fixed shape, and at least one of a size, the number or a depth of the holes or slits formed in the magnetic-material is suitably adjusted thus adjusting the sizes of the non-magnetic region with high accuracy. 
     The holes or the slits may be formed by melting a ferro magnetic-material using a wire electric discharge machine or by forming cut grooves using a diamond cutter. 
     Alternatively, the size of the non-magnetic region of the third permanent magnet  13  or the fourth permanent magnet  14  may be adjusted as follows. That is, a plurality of granular magnetic materials having a lump shape which is sufficiently small compared to a volume of the third permanent magnet  13  or the fourth permanent magnet  14  and a non-magnetic material are mixed to each other, the mixture is formed into an integral body having a predetermined shape, the magnetic materials are magnetized to saturated residual magnetic flux density by an external magnetic field having predetermined intensity thus forming the permanent magnet, and a mixing ratio of the non-magnetic material and the granular magnetic materials is adjusted so as to adjust the size of the non-magnetic region. 
     As the non-magnetic material used in this embodiment, a non-magnetic resin material can be used. Alternatively, a ceramics material such as silicon dioxide may be used as the non-magnetic material, and such a non-magnetic material maybe sintered together with the granular magnetic materials to form an integral body. 
     In this case, it is desirable that the granular magnetic materials have a sufficiently small size compared to a volume of the third permanent magnet  13  or the fourth permanent magnet  14 , and it is also desirable that the size of the granular magnetic materials is smaller than at least one tenth of the volume of the third permanent magnet  13  and the fourth permanent magnet  14 . It is more desirable to set the size of the granular magnetic materials to one hundredth or less of the volume of the third permanent magnet  13  and the fourth permanent magnet  14 . It is not always necessary to form the granular magnetic materials into a rectangular parallelepiped shape and may be formed in a suitable shape. 
     Further, when the non-magnetic resin material is used as the non-magnetic material, a predetermined quantity of granular magnetic materials and a predetermined quantity of resin material are mixed to each other, the mixture is filled into a molding vessel which is formed in conformity with a shape of the third permanent magnet  13  or the fourth permanent magnet  14 , and the resin is solidified. Thereafter, the magnetic materials are magnetized to the saturated residual magnetic flux density by an external magnetic field having predetermined intensity thus forming the third permanent magnet  13  or the fourth permanent magnet  14  having a predetermined shape. By solidifying the resin before the magnetic materials are magnetized by the external magnetic field having predetermined intensity, it is possible to form the third permanent magnet  13  or the fourth permanent magnet  14  in which the materials which become the permanent magnet are uniformly distributed. 
     Alternatively, in the adjustment of the size of the non-magnetic region, instead of using the granular magnetic materials and the resin material, a plurality of permanent magnet blocks having a predetermined size and a rectangular parallelepiped shape and a plurality of non-magnetic-material blocks having the same shape as the permanent magnet blocks may be used, and the non-magnetic-material blocks and the permanent magnet blocks may be integrally bonded to each other using an adhesive agent or the like thus forming the third permanent magnet  13  and the fourth permanent magnet  14 . 
     In this case, by adjusting a content ratio between the permanent magnet blocks and the non-magnetic-material blocks, the size of the non-magnetic region can be easily adjusted. 
     Here, in forming the third permanent magnet  13  or the fourth permanent magnet  14  by integrally bonding the permanent magnet blocks and the non-magnetic-material blocks, the permanent magnet blocks may be preliminarily magnetized to the saturated residual magnetic flux density by an external magnetic field having predetermined intensity or the permanent magnet blocks may be bonded to the non-magnetic-material blocks to be formed into an integral body having a predetermined shape and, thereafter, the integral body may be magnetized to the saturated residual magnetic flux density by an external magnetic field having predetermined intensity. Although the permanent magnet blocks and the non-magnetic-material blocks are formed in a rectangular parallelepiped shape for easing the formation of these blocks in this embodiment, these blocks are not always limited to a rectangular parallelepiped shape, and may be formed in a suitable shape. 
     In this manner, by forming the non-magnetic-material region where the magnetic material is not present in the third permanent magnet  13  and the fourth permanent magnet  14 , and by adjusting the size of the non-magnetic-material region, the apparent residual magnetic flux density of the third permanent magnet  13  and the fourth permanent magnet  14  can be easily adjusted thus forming the third permanent magnet  13  and the fourth permanent magnet  14  having the predetermined residual magnetic flux density with extremely high accuracy. Accordingly, the target region T having the uniform and stable magnetic flux density can be easily formed and, at the same time, the size of the target region T can be easily adjusted. 
     Further, the third permanent magnet  13  and the fourth permanent magnet  14  are respectively magnetized to the saturated residual magnetic flux density and hence, it is possible to prevent the residual magnetic flux density from being changed due to other permanent magnets such as the first permanent magnet  11  and the second permanent magnet  12 . 
       FIG. 8  is a plan view of a magnetic field generator A 1 ′ according to a modification of the first embodiment,  FIG. 9  is an end surface view as viewed from a line Y 1 -Y 1  in  FIG. 8 , and  FIG. 10  is an end surface view as viewed from a line Y 2 -Y 2  in  FIG. 8 . 
     In the magnetic field generator A 1 ′ of the modification, in place of the above-mentioned two permanent magnets consisting of the first permanent magnet  11  and the second permanent magnet  12 , as shown in  FIG. 8 , one cylindrical permanent magnet  30  which is formed in an integral cylindrical shape is used. In this cylindrical permanent magnet  30 , two third permanent magnets  33 ,  33  and two fourth permanent magnets  34 ,  34  are arranged in a hollow center portion. 
     The cylindrical permanent magnet  30  is magnetized along the reference direction. The direction of magnetization of the third permanent magnet  33  is directed in the direction which makes a predetermined angle α with respect to the reference direction, and particularly directed toward a main surface  10   a  side. The direction of magnetization of the fourth permanent magnet  34  is directed in the direction which makes a predetermined angle α with respect to the reference direction, and particularly directed toward a side opposite to the main surface  10   a  side. 
     Two third permanent magnets  33 ,  33  are respectively arranged in a mirror symmetry with respect to a symmetry plane MP which passes the center of the cylindrical permanent magnet  30  and is arranged parallel to the reference direction and, at the same time, two fourth permanent magnets  34 ,  34  are also respectively arranged in a mirror symmetry with respect to the symmetry plane MP which passes the center of the cylindrical permanent magnet  30  and is arranged parallel to the reference direction. 
     Further, first inner permanent magnets  35 - 1  are respectively formed on symmetry-plane-MP sides of the third permanent magnets  33  and, at the same time, first outer permanent magnets  36 - 1  are respectively formed on sides opposite to the first inner permanent magnets  35 - 1  with the third permanent magnets  33  sandwiched therebetween. 
     In the same manner, second inner permanent magnets  35 - 2  are respectively formed on symmetry-plane-MP sides of the fourth permanent magnets  34  and, at the same time, second outer permanent magnets  36 - 2  are respectively formed on sides opposite to the second inner permanent magnets  35 - 2  with the fourth permanent magnets  34  sandwiched therebetween. 
     With respect to the direction of magnetization in the first inner permanent magnets  35 - 1 , the first outer permanent magnets  36 - 1 , the second inner permanent magnets  35 - 2  and the second outer permanent magnets  36 - 2 , as indicated by arrows in  FIG. 9  and  FIG. 10 , the direction of magnetization is directed in the direction of a second component in the first inner permanent magnets  35 - 1  and the second outer permanent magnets  36 - 2 , and is directed in the direction opposite to the second component in the first outer permanent magnets  36 - 1  and second inner permanent magnets  35 - 2 . 
     In the magnetic field generator A 1 ′ having such constitution, uniformity in the depth direction in the target region T can be enhanced thus enhancing the detection accuracy of a stereoscopic object. 
     To be more specific, the cylindrical permanent magnet  30  is formed in a quadrangular cylindrical shape in which an outer circumference of the permanent magnet  30  has a quadrangular shape with each side thereof set to 40 cm and the permanent magnet  30  has a quadrangular opening with each side thereof set to 20 cm inside the outer circumference. A size of the cylindrical permanent magnet  30  in the depth direction is set to 20 cm. 
     Sizes of the third permanent magnet  33  and the fourth permanent magnet  34  are respectively set to a longitudinal size of 5 cm ×a lateral size of 5 cm×a depth of 10 cm, sizes of the first inner permanent magnets  35 - 1  and the second inner permanent magnets  35 - 2  are respectively set to a longitudinal size of 0.8 cm×a lateral size of 0.8 cm×a depth of 0.8 cm, and the sizes of the first outer permanent magnets  36 - 1  and the second outer permanent magnets  36 - 2  are respectively set to a longitudinal size of 2 cm×a lateral size of 4.8 cm×a depth of 2.2 cm. 
     Two first inner permanent magnets  35 - 1  are arranged adjacent to each other, and may be integrally formed depending on a case. In the same manner, two second inner permanent magnets  35 - 2  are also arranged adjacent to each other, and may be integrally formed depending on a case. A distance of approximately 4 mm is set between the first inner permanent magnets  35 - 1  and the second inner permanent magnets  35 - 2  arranged adjacent to each other. 
     Further, as shown in  FIG. 9  and  FIG. 10 , with respect to the third permanent magnets  33 , the fourth permanent magnets  34 , the first inner permanent magnets  35 - 1 , the second inner permanent magnets  35 - 2 , the first outer permanent magnets  36 - 1  and the second outer permanent magnets  36 - 2  which are arranged in the center portion of the cylindrical permanent magnet  30 , an end surface of each permanent magnet on a main surface  10   a  side of the casing  10  is arranged to be retracted from an end surface of the cylindrical permanent magnet  30  on the main surface  10   a  side of the casing  10 . In this embodiment, the end surface of each permanent magnet on the main surface  10   a  side of the casing  10  is retracted by approximately 3 mm. 
     The residual magnetic flux density of the cylindrical permanent magnet  30  is set to 1.38 T, the residual magnetic flux densities of the third permanent magnets  13  and the fourth permanent magnets  14  are set to 1.30 T, the residual magnetic flux densities of the first inner permanent magnets  35 - 1  and the second inner permanent magnets  35 - 2  are set to 0.07 T, the residual magnetic flux densities of the first outer permanent magnets  36 - 1  and the second outer permanent magnets  36 - 2  are set to 0.33 T, and a value of the angle α is set to 14.8°. 
     In this embodiment, the residual magnetic flux densities of the third permanent magnets  13 , the fourth permanent magnets  14 , the first inner permanent magnets  35 - 1 , the second inner permanent magnets  35 - 2 , the first outer permanent magnet  36 - 1  and the second outer permanent magnets  36 - 2  are set to desired apparent residual magnetic flux densities by adjusting the size of the non-magnetic region as described previously. 
     A following table shows the uniformity (ppm) of magnetic flux densities of estimated regions under the following conditions. That is, the end surface of the cylindrical permanent magnet  30  shown in  FIG. 8  on the main surface  10   a  side of the casing  10  is set as an X-Y plane. Using the center of the cylindrical permanent magnet  30  on the X-Y plane as an origin, an x axis is estimated on the symmetry plane MP, a y axis is estimated orthogonal to the x axis, and a z axis is estimated in the direction toward the target region from the X-Y plane. By setting a position determined by X=0 cm, Y=0 cm and Z=5.5 cm as the center, a range Δx in the X-axis direction, a range Δy in the Y-axis direction, and a range Δz in the Z-axis direction are set to various values. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Δx [cm] 
                 Δy [cm] 
                 ΔZ [cm] 
                 uniformity(ppm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1.0 
                 1.0 
                 0.1 
                 28 
               
               
                   
                 1.0 
                 1.0 
                 0.2 
                 40 
               
               
                   
                 1.0 
                 1.0 
                 0.3 
                 58 
               
               
                   
                 1.0 
                 1.0 
                 0.4 
                 83 
               
               
                   
                 1.0 
                 1.0 
                 0.5 
                 110 
               
               
                   
                 1.0 
                 1.0 
                 0.6 
                 140 
               
               
                   
                 1.0 
                 1.0 
                 0.7 
                 171 
               
               
                   
                 1.0 
                 1.0 
                 0.8 
                 206 
               
               
                   
                 1.0 
                 1.0 
                 0.9 
                 245 
               
               
                   
                 1.0 
                 1.0 
                 1.0 
                 288 
               
               
                   
                 2.0 
                 2.0 
                 0.1 
                 177 
               
               
                   
                 2.0 
                 2.0 
                 0.2 
                 195 
               
               
                   
                 2.0 
                 2.0 
                 0.3 
                 221 
               
               
                   
                 2.0 
                 2.0 
                 0.4 
                 244 
               
               
                   
                 2.0 
                 2.0 
                 0.5 
                 275 
               
               
                   
                 2.0 
                 2.0 
                 0.6 
                 332 
               
               
                   
                 2.0 
                 2.0 
                 0.7 
                 400 
               
               
                   
                 2.0 
                 2.0 
                 0.8 
                 470 
               
               
                   
                 2.0 
                 2.0 
                 0.9 
                 537 
               
               
                   
                 2.0 
                 2.0 
                 1.0 
                 613 
               
               
                   
                   
               
            
           
         
       
     
     It is understood from Table 1 that this embodiment can acquire uniformity which is sufficiently available for the detection of the relatively minute structure such as an eye, an ear, a nose or a teeth. 
       FIG. 11  is a longitudinal cross-sectional view of a magnetic field generator A 2  of a second embodiment. This magnetic field generator A 2  has the substantially same constitution as the magnetic field generator A 1  of the first embodiment, while in this embodiment, a fifth permanent magnet  15  is provided between the first permanent magnet  11  and the third permanent magnet  13  of the magnetic field generator A 1  of the first embodiment, and a sixth permanent magnet  16  is provided between the fourth permanent magnet  14  and the second permanent magnet  12  of the magnetic field generator A 1  of the first embodiment. Accordingly, constitutional parts identical with the corresponding constitutional parts of the magnetic field generator A 1  of the first embodiment are given same symbols and their repeated explanation is omitted. 
     The fifth permanent magnet  15  and the sixth permanent magnet  16  have the same magnitude of magnetization. Further, the direction of magnetization of the fifth permanent magnet  15  is, as indicated by an arrow in  FIG. 11 , directed in the direction parallel to the second component of the third permanent magnet  13 , and the direction of magnetization of the sixth permanent magnet  16  is, as indicated by an arrow in  FIG. 11 , directed in the direction parallel to the inversed second component of the fourth permanent magnet  13 . 
     In this manner, by providing the fifth permanent magnet  15  between the first permanent magnet  11  and the third permanent magnet  13  and, at the same time, by providing the sixth permanent magnet  16  between the fourth permanent magnet  14  and the second permanent magnet  12 , the uniformity of the magnetic flux density in a target region T generated by the magnetic field generator A 2  can be enhanced and, at the same time, a size of the target region T can be further increased. 
     Here, both of magnitudes of magnetization of the fifth permanent magnet  15  and the sixth permanent magnet  16  largely influence the size of the region having the uniform magnetic flux density and hence, it is desirable to set the sizes of these magnetization to predetermined sizes of magnetization with accuracy as high as possible. Accordingly, the size of non-magnetic region is adjusted preliminarily before magnetization such that the desired apparent residual magnetic flux densities can be acquired after magnetization. 
     Further, in this embodiment, the fifth permanent magnet  15  is arranged to be in contact with the third permanent magnet  13 . However, it is not always necessary to arrange the fifth permanent magnet  15  in contact with the third permanent magnet  13 . That is, by adjusting the magnitude of magnetization and the direction of the magnetization of the fifth permanent magnet  15 , the fifth permanent magnet may be arranged at a suitable position between the first permanent magnet  11  and the third permanent magnet  13 . 
     In the same manner, although the sixth permanent magnet is arranged to be in contact with the fourth permanent magnet  14 , it is not always necessary to arrange the sixth permanent magnet  16  in contact with the fourth permanent magnet  14 . That is, by adjusting the magnitude of magnetization and the direction of the magnetization of the sixth permanent magnet  16 , the sixth permanent magnet  16  may be arranged at a suitable position between the fourth permanent magnet  14  and the second permanent magnet  12 . 
     Particularly, by setting the components of magnetization of the fifth permanent magnet  15  and the sixth permanent magnet  16  in the direction parallel to the reference direction to negative values, that is, by allowing the fifth permanent magnet  15  and the sixth permanent magnet  16  to have the components in the direction antiparallel to the reference direction, uniformity of magnetic flux density in the target region T can be enhanced and, at the same time, the size of the target region T can be further increased. 
     Alternatively, by setting the components of magnetization of the fifth permanent magnet  15  and the sixth permanent magnet  16  in the direction parallel to the reference direction to positive values, that is, by allowing the fifth permanent magnet  15  and the sixth permanent magnet  16  to have the components in the direction parallel to the reference direction, the size of the target region T can be further increased. 
     As shown in  FIG. 11 , in the magnetic field generator A 2  in which the direction of magnetization of the fifth permanent magnet  15  is directed in the direction parallel to the second component of the third permanent magnet  13  and the direction of magnetization of the sixth permanent magnet  16  is directed in the direction parallel to the inverted second component of the fourth permanent magnet  14 , it is confirmed by a numerical value analysis that a magnetic field having the magnetic flux density distribution in  FIG. 12  can be generated. Here, sizes of the first permanent magnet  11  and the second permanent magnet  12  are respectively set to have a cross section of 10 cm×20 cm shown in  FIG. 11 , sizes of the third permanent magnet  13  and the fourth permanent magnet  14  are respectively set to have a cross section of 5 cm×10 cm shown in  FIG. 11 , and sizes of the fifth permanent magnet 15 and the sixth permanent magnet  16  are respectively set to have a cross section of 2 cm×2 cm shown in  FIG. 8 . A distance of approximately 3 cm is provided between the first permanent magnet  11  and the fifth permanent magnet  15  as well as between the sixth permanent magnet  16  and the second permanent magnet  12 . Residual magnetic flux densities of the first permanent magnet  11  and the second permanent magnet  12  are set to 1.38 T, residual magnetic flux densities of the third permanent magnet  13  and the fourth permanent magnet  14  are set to 1.05 T, a value of an angle α is set to a value of 16.5°, and residual magnetic flux densities of the fifth permanent magnet  15  and the sixth permanent magnet  16  is set to 0.06 T. 
       FIG. 13  shows a magnitude of magnetic flux density in the Z axis direction when X=0 in  FIG. 12 .  FIG. 14  shows a magnitude of magnetic flux density in the X axis direction from X=0 when Z=5.75 cm, 6.0 cm, 6.25 cm. It is understood from  FIG. 13  and  FIG. 14  that the target region T having uniform magnetic flux density is formed at a position approximately 6 cm from the surface of the casing  10  opposite to the main surface  10   a  which constitutes one end peripheries of the first to forth permanent magnet  11  to  14 . 
     Particularly, the uniformity in the target region T where X is set to −1.0 cm&lt;X&lt;1.0 cm and Z is set to 5.75 cm&lt;Z&lt;6.25 cm is 92 ppm. That is, for example, with respect to a region which becomes necessary for realizing the observation of a part such as an eye, an ear, a tooth or the like using a nuclear magnetic resonance device, it is possible to form a region having the uniformity of 100 ppm. Such a magnetic field generator A 2  and the magnetic field generator A 1  of the first embodiment can be used as a magnetic field generator of a nuclear magnetic resonance device. 
     Here, in such a magnetic field generator, the first to sixth permanent magnets  11  to  16  may be, as shown in  FIG. 15 , formed of permanent magnets elongated in the cross-sectional direction of  FIG. 1  and  FIG. 11 , or the first to sixth permanent magnets  11  to  16  which are respectively set to predetermined lengths may be, as shown in  FIG. 16 , arranged in series in the longitudinal direction of  FIG. 15 . 
     In this manner, according to this magnetic field generator, with the use of the first to sixth permanent magnets  11  to  16  which are elongated in a pseudo manner by forming these magnets in series, non-uniformity of a magnetic field in the longitudinal direction can be eliminated thus generating a uniform magnetic field also in the longitudinal direction. 
     Further, the magnetic field generator may use not only the first to sixth permanent magnets  11  to  16  which are respectively formed in a rectangular parallelepiped shape as viewed in a plan view as shown in  FIG. 15  and  FIG. 16  but also first to sixth permanent magnets  11 ′ to  16 ′ which are shaped in a circular shape toward the main surface  10   a  as shown in  FIG. 17 . 
     That is, as shown in  FIG. 17 , the first to sixth permanent magnets  11 ′ to  16 ′ which are arranged in order of the first permanent magnet  11 ′, the fifth permanent magnet  15 ′, the third permanent magnet  13 ′, the fourth permanent magnet  14 ′, the sixth permanent magnet  16 ′ and the second permanent magnet  12 ′ along the reference direction have side surfaces thereof formed into predetermined curved shapes such that side surfaces of the first to sixth permanent magnets  11 ′ to  16 ′ on a main surface  10   a  side form a circle as a whole. 
     Further, as shown in  FIG. 18 , in the magnetic field generator, the first permanent magnet  11  and the second permanent magnet may be formed of a cylindrical permanent magnet  17 ″ having an integral cylindrical shape, a third permanent magnet  13 ″ and a fourth permanent magnet  14 ″ formed of a semicircular columnar body having a semicircular transverse cross-sectional shape are arranged in a hollow center portion of the cylindrical permanent magnet  17 ″, a fifth permanent magnet  15 ″ having a semicircular shape is arranged along an outer peripheral surface of the third permanent magnet  13 ″, and a sixth permanent magnet  16 ″ having a semicircular shape may be arranged along an outer peripheral surface of the fourth permanent magnet  14 ″. 
     Also in this case, it is desirable that the integral cylindrical permanent magnet  17 ″ is magnetized along the reference direction, the directions of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14  respectively make an angle α of 10.8° with respect to the reference direction, the direction of magnetization of the fifth permanent magnet  15 ″ is directed toward a target region T side, and the direction of magnetization of the sixth permanent magnet  16 ″ is directed in the direction opposite to the direction of magnetization of the fifth permanent magnet  15 ″. 
     Further, as a modification of this embodiment, as shown in  FIG. 19 , the magnetic field generator may use a quadrangular cylindrical permanent magnet  27  in place of the cylindrical permanent magnet  17 ″. 
     Further, a third permanent magnet  23  and a fourth permanent magnet  24  which respectively have a rectangular parallelepiped shape may be arranged in a hollow center portion of the quadrangular cylindrical permanent magnet  27 , a fifth permanent magnet  25  which is bent in a U shape along an outer peripheral surface of the third permanent magnet  23  may be arranged on an outer peripheral surface of the third permanent magnet  23 , and a sixth permanent magnet  26  which is bent in a U shape along an outer peripheral surface of the fourth permanent magnet  24  may be arranged on an outer peripheral surface of the fourth permanent magnet  24 . 
     Particularly, the third permanent magnet  23  is constituted of two permanent magnets  23 - 1 ,  23 - 2  which are arranged in a mirror symmetry with respect to a symmetry plane parallel to the reference direction, and the fourth permanent magnet  24  is constituted of two permanent magnets  24 - 1 ,  24 - 2  which are arranged in a mirror symmetry with respect to a symmetry plane parallel to the reference direction. In this embodiment, a contact surface between two permanent magnets  23 - 1 ,  23 - 2  which constitute the third permanent magnet  23 , and a contact surface between two permanent magnets  24 - 1 ,  24 - 2  which constitute the fourth permanent magnet  24  respectively form the symmetry planes. It is not always necessary that two permanent magnets  23 - 1 ,  23 - 2  which constitute the third permanent magnet  23  are brought into contact with each other on the contact surface, and a predetermined distance may be provided between two permanent magnets  23 - 1 ,  23 - 2 . In the same manner, it is not always necessary that two permanent magnets  24 - 1 ,  24 - 2  which constitute the fourth permanent magnet  24  are brought into contact with each other on the contact surface, and a predetermined distance may be provided between two permanent magnets  24 - 1 ,  24 - 2 . 
     Two permanent magnets  23 - 1 ,  23 - 2  which constitute the third permanent magnet  23 , as shown in  FIG. 20  which is a plan view of the third permanent magnet  23  and the fourth permanent magnet  24 , include a magnetization component which constitutes a first component M 1  which is directed in the direction parallel to the reference direction and a magnetization component which constitutes a first orthogonal component C 1  which is directed in the direction orthogonal to the symmetry plane and also is directed toward the symmetry plane. 
     Two permanent magnets  24 - 1 ,  24 - 2  which constitute the fourth permanent magnet  24 , as shown in  FIG. 20  which is the plan view of the third permanent magnet  23  and the fourth permanent magnet  24 , include a magnetization component which constitutes a first component M 1  which is directed in the direction parallel to the reference direction and a magnetization component which constitutes a second orthogonal component C 2  which is directed in the direction orthogonal to the symmetry plane and also is directed in the direction opposite to the symmetry plane. 
     In this manner, by constituting the third permanent magnet  23  using two permanent magnets  23 - 1 ,  23 - 2  and by constituting the fourth permanent magnet  24  using two permanent magnets  24 - 1 ,  24 - 2 , the uniformity in the target region can be enhanced. 
     Here, it is desirable that the quadrangular cylindrical permanent magnet  27  is magnetized in the reference direction, the directions of magnetization of the third permanent magnet  13  and the fourth permanent magnet  14  respectively make an angle α of 10.8° with respect to the reference direction, the direction of magnetization of the fifth permanent magnet  25  is directed toward a target region side, and the direction of magnetization of the sixth permanent magnet  26  is directed in the direction opposite to the direction of magnetization of the fifth permanent magnet  25 . Further, as shown in  FIG. 21 , it is desirable that the directions of magnetization of the respective permanent magnets  23 ,  24 ,  25 ,  26 , and  27  on a cross section which passes the center of the quadrangular cylindrical permanent magnet  27  are directed in the same directions as the directions of magnetization of the respective permanent magnets  11 ,  12 ,  13 ,  14 ,  15 , and  16  shown in  FIG. 5 . 
     Further, in place of the quadrangular cylindrical permanent magnet  27 , as shown in  FIG. 22 , a first permanent magnet  21  may be constituted of two permanent magnets  21 - 1 ,  21 - 2  which are arranged in a mirror symmetry with respect to a symmetry plane parallel to the reference direction, a second permanent magnet  22  may be constituted of two permanent magnets  22 - 1 ,  22 - 2  which are arranged in a mirror symmetry with respect to the symmetry plane, two permanent magnets  21 - 1 ,  21 - 2  which constitute the first permanent magnet  21  may have magnetic components which are respectively directed in the direction orthogonal to the symmetry plane and are directed toward the symmetry plane, and two permanent magnets  22 - 1 ,  22 - 2  which constitute the second permanent magnet  22  may have magnetic components which are respectively directed in the direction orthogonal to the symmetry plane and are directed in the direction opposite to the symmetry plane. 
     By constituting the first permanent magnet  21  using two permanent magnets  21 - 1 ,  21 - 2  and by constituting the second permanent magnet  22  using two permanent magnets  22 - 1 ,  22 - 2 , the permanent magnet can be miniaturized thus realizing the reduction of a manufacturing cost. 
     In the above-mentioned magnetic field generator A 2 , it is not always necessary to bring the third to sixth permanent magnets  13  to  16  into contact with the surface of the casing  10  opposite to the main surface  10   a , and the third to sixth permanent magnets  13  to  16  may be spaced apart from the surface of the casing  10  opposite to the main surface  10   a  by a predetermined distance. Particularly, when the magnetic field generator A 2  is used as a nuclear magnetic resonance device, by providing a reception coil or a transmission coil of the nuclear magnetic resonance device between the third to sixth permanent magnets  13  to  16  which are spaced apart from the surface of the casing  10  opposite to the main surface  10   a  and the main surface  10   a , the nuclear magnetic resonance device can be made in a compact shape. 
     That is, in a magnetic field generator A 3  for a nuclear magnetic resonance device shown in  FIG. 23 , by arranging the reception coil  18  between the surface of the casing  10  opposite to the main surface  10   a  and the third to sixth permanent magnets  13  to  16  and by arranging the transmission coil  19  in the inside of the casing  10 , the nuclear magnetic resonance device is formed in a more compact shape. The magnetic field generator A 3  is mounted on a stand not shown in the drawing. 
     The magnetic field generator A 3  can form the target region T having the uniform magnetic flux density using six or four permanent magnets and hence, it is possible to provide a relatively light-weighted magnetic field generator, and the magnetic field generator A 3  can be used in a state that the magnetic field generator A 3  is mounted on the relatively simple stand whereby it is possible to provide a nuclear magnetic resonance device which can be manufactured at a low cost and exhibits favorable handling property. 
     Further, a magnetic field which is generated by the magnetic field generator A 3  exhibits the magnetic flux density directed in the direction parallel to the main surface  10   a  and hence, it is possible to enhance detection sensitivity of a signal in the nuclear magnetic resonance device. 
     INDUSTRIAL APPLICABILITY 
     According to the magnetic field generator and the nuclear magnetic resonance device which includes such a magnetic field generator of the present invention, it is possible to provide the magnetic field generator which has the magnetic circuit of an open magnetic path and can form the region having the more uniform and stable magnetic flux density and hence, by applying the magnetic field generator to the nuclear magnetic resonance device, it is possible to provide the nuclear magnetic resonance device which exhibits high detection accuracy of a predetermined signal thus realizing the inspection of high accuracy. 
     Particularly, the magnetic field generator can be formed in a compact shape and hence, the magnetic field generator can be manufactured at a low cost and, at the same time, portability of the magnetic field generator can be enhanced. Accordingly, a magnetic field generator can be used not only in the observation of a part relatively close to a surface of a body such as an eye, an ear, a nose, a tooth or the like but also in a non-destructive inspection of a structural body such as a concrete structural body or in an inspection carried out underground. Particularly, the magnetic field generator can detect a plastic-made mine so that the magnetic field generator can detect an object which cannot be detected by a conventional metal detector.