hereunder , a metal gasket according to embodiments of the present invention will be explained with reference to the attached drawings . incidentally , fig1 - 24 are schematic explanatory views in which sizes of a seal target bore , bolt holes and half beads ; widths of a middle plate and a surface pressure assistance plate ; and thicknesses of a metal plate , middle plate and the surface pressure assistance plate ; and sizes of depressions and projections are different from actual ones and enlarged for the sake of explanation . also , for the sake of simplicity , hereunder , the seal target bore will be explained as one seal target bore . however , the present invention can be applied even when multiple kinds of seal target bores such as gas circulation holes , combustion chamber holes ( bore holes ), water holes , or oil holes , are respectively provided with multiple numbers , such as a gasket for an exhaust manifold for a multiple cylinder engine or a cylinder head gasket . incidentally , the following “ outside ” and “ inside ” are terms with respect to a thickness direction of the gasket , and the horizontal direction of the gasket is described as an “ outer perimeter side ” and an “ inner perimeter side ”. as shown in fig1 , the metal gasket 1 of the embodiments of the present invention is formed of multiple metal plates ( metal composition plates ) made of soft steel , annealed stainless ( annealed material ), or stainless material ( spring steel ). also , the metal gasket 1 is produced in a shape corresponding to the shape of a member clamping the metal gasket 1 , and is provided with a seal target bore 2 and bolt holes 3 for inserting tightening bolts . in the first embodiment shown in fig1 - 5 , this metal gasket 1 has a first metal plate 10 and a second metal plate 20 arranged symmetrically , which clamp a middle plate 40 . also , a surface pressure assistance plate 30 forming depressions and projections with respect to a thickness direction of the metal gasket 1 , is disposed on the outer perimeter side of the middle plate 40 , and inside between sloping parts 11 b , 21 b of half beads 11 , 21 and flat parts 11 c , 21 c on the outer perimeter side . more specifically , an inner perimeter side end of the surface pressure assistance plate 30 is disposed inside the sloping parts 11 b , 21 b . for example , the first metal plate 10 and the second metal plate 20 are made of soft steel or annealed stainless ( annealed material ), and provided with the seal target bore 2 and the bolt holes 3 surrounding the seal target bore 2 . also , the first metal plate 10 and the second metal plate 20 respectively provide the half beads 11 , 21 surrounding the seal target bore 2 and extending outside with respect to the thickness direction of the gasket in a direction away from the seal target bore 2 . also , the first metal plate 10 and the second metal plate 20 are symmetrically disposed clamping the middle plate 40 . the middle plate 40 is made of soft steel , annealed stainless ( annealed material ), or stainless material ( spring steel ), and as shown in fig2 , disposed in a ring shape ( circularity ) around the seal target bore 2 . as shown in fig3 , an inner perimeter side end 40 a facing the seal target bore 2 is arranged inside flat parts 11 a , 21 a on the inner perimeter side ( seal target bore 2 side ) of the half beads 11 , 21 . the middle plate 40 prevents liquid from entering between the half beads 11 , 21 and adjusts a thickness . therefore , the middle plate 40 is formed in a sheet of ring - shaped continuous plate in a circumferential direction . as shown in fig2 , the surface pressure assistance plate 30 is disposed in a ring shape , and as shown in fig4 , 5 , forms the depressions and the projections with respect to the thickness direction . the surface pressure assistance plate 30 has elasticity relative to a compressive direction of the half beads 11 , 21 of the metal gasket 1 . when the half beads 11 , 21 are compressed , as shown in fig5 , the surface pressure assistance plate 30 abuts against the sloping parts 11 b , 21 b of the half beads 11 , 21 , and provides a cushion effect , thereby adjusting the surface pressure of the half beads 11 , 21 . the surface pressure assistance plate 30 is made of stainless material ( spring steel ) and the like , and as shown in fig4 , 12 , 15 , 16 , the depressions and the projections may be formed with straight lines such as a trapezoid . however , as shown in fig9 , 14 , 17 , 18 , 19 , the depressions and the projections may be formed with curved lines such as a circular shape or a sine shape . in addition , as shown in fig4 , 5 , 9 , 12 , 14 , 15 , 17 , 18 , the depressions and the projections may be formed by a single step , or as shown in fig1 , 19 , formed by multiple steps . moreover , the depressions and the projections may or may not be axisymmetric with respect to the thickness direction . also , with respect to the perimeter direction of the seal target bore 2 , the depressions and the projections may be formed in a continuous wave pattern and provided all around the seal target bore 2 , or may be formed in a continuous wave pattern ; a single projection ; or a single depression , and the depressions and the projections may be formed in only a part wherein a surface pressure adjustment is required . also , shapes and sizes of the depressions and the projections may also be changed according to the degree of the surface pressure adjustment . moreover , the surface pressure assistance plate 30 itself may be disposed all around the seal target bore 2 , or may be disposed only in the part wherein the surface pressure adjustment is required . basically , the depressions and the projections need only to have appropriate elasticity relative to the compressive direction of the metal gasket not to cause creep relaxation . the depressions and the projections can be easily formed by a pressing process and the like . also , as shown in fig6 , the direction of top parts 30 a of the depressions and the projections of the surface pressure assistance plate 30 is a direction intersecting with the half beads 11 , 21 , preferably , 80 - 100 °, more preferably , 90 ° ( perpendicular ), so that an elastic effect by the depressions and the projections of the surface pressure assistance plate 30 can be used more efficiently compared to the case of arranging the top parts of the depressions and the projections parallel to the half beads 11 , 21 . also , when the top parts of the depressions and the projections are arranged in the intersecting direction , very narrow ( linear ) hit of corner parts of the half beads 11 , 21 can be made wide ( planar ) hit by abutting the depressions and the projections of the surface pressure assistance plate 30 against the slop parts 11 b , 21 b of the half beads 11 , 21 , so that the local surface pressure can be decreased . as a result , an abutting part in a member in which the half beads 11 , 21 abut , can be prevented from being scratched . incidentally , as shown in fig7 , in the surface pressure assistance plate 30 , when the direction of the top parts 30 a of the depressions and the projections is made in one direction , an evenly spread surface pressure cannot be achieved as compared to the case wherein the top parts 30 a of the depressions and the projections are arranged in the direction intersecting with the half beads 11 , 21 . however , in this case , since the depressions and the projections are simplified , they can be produced more easily . with respect to the arrangement of the intersecting direction , the surface pressure adjustment changes even by an intersecting angle , so that if the intersecting angle is changed by a required compressive elasticity ( spring force ), a precise surface pressure adjustment can be achieved . more specifically , in a part of perimeters of the half beads 11 , 21 , or all around the half beads 11 , 21 , the direction of the top parts of the depressions and the projections of the surface pressure assistance plate 30 is the direction intersecting with the half beads 11 , 21 . when the direction of the top parts of the depressions and the projections of the surface pressure assistance plate 30 is the direction intersecting with the half beads 11 , 21 all around the half beads 11 , 21 , for example , when the seal target bore 2 is a circle , the top parts ( valleys and mountains ) of the depressions and the projections are arranged radially . also , the surface pressure assistance plate 30 may be formed as a continuous ring in a perimeter direction of the seal target bore 2 . however , since the function is to adjust the surface pressure , the surface pressure assistance plate 30 does not necessarily need to be provided in a continuous integral part all around the perimeter direction , and divided multiple layers of surface pressure assistance plates 30 may be arranged to contact each other , or may be disposed separately only in parts wherein the surface pressure adjustment is required . more specifically , in the part of the perimeter of the half beads 11 , 21 or all around the half beads 11 , 21 , the surface pressure assistance plates 30 forming the depressions and the projections with respect to the thickness direction are disposed inside the half beads 11 , 21 . according to the structure , the degree of the surface pressure adjustment can be changed with or without the surface pressure assistance plate 30 , by changes of shape and size ( height , width and so on ) of the depressions and the projections , a length of the top parts of the depressions and the projections ( width of the surface pressure assistance plate 30 ), or an intersecting angle between the direction of the top parts of the depressions and the projections and the half beads 11 , 21 . accordingly , the surface pressure can be easily adjusted very precisely . as shown in fig8 , 9 , the metal gasket according to the second embodiment of the invention differs from the metal gasket according to the first embodiment , because the middle plate 40 and the surface pressure assistance plate 30 are formed in one sheet of surface pressure assistance plate 30 . the other structure of the metal gasket of the second embodiment is the same as that of the metal gasket of the first embodiment . incidentally , as shown in fig9 , the surface pressure assistance plate 30 is made to be flat on the inner perimeter side , and forms curved depressions and projections on the outer perimeter side . in the second embodiment , the same plate has both functions of the middle plate 40 which is required for a sealing effect and the surface pressure assistance plate 30 which is required for an elastic effect . accordingly , a material cannot be used separately . however , the second embodiment can be produced more easily compared to the first embodiment because the middle plate 40 is eliminated and a width of the surface pressure assistance plate 30 increases . as shown in fig1 , the metal gasket according to the third embodiment of the invention differs from the metal gasket according to the first embodiment because the middle plate 40 is eliminated . the other structure of the third embodiment is the same as that of the metal gasket of the first embodiment . due to such a structure , the seal quality cannot be improved by the middle plate 40 . however , the metal gasket of the third embodiment can reduce the weight and size , and be produced more easily than the metal gasket of the first embodiment . as shown in fig1 , 12 , the metal gasket according to the fourth embodiment of the invention laminates the first metal plate 10 , forming the half bead 11 surrounding the seal target bore 2 and expanding outside in the direction away from the seal target bore 2 ; the middle plate 40 disposed on the inner perimeter side of the half bead 11 , i . e ., inside the flat part 11 a on the seal target bore 2 side ; the surface pressure assistance plate 30 forming the depressions and the projections and disposed on the outer perimeter side and inside the half bead 11 between the sloping part 11 b of the half bead 11 and the flat part 11 c on the outer perimeter side ; and the flat second metal plate 20 . the metal gasket of the fourth embodiment differs from the metal gasket of the first embodiment , because the half bead 21 is not formed on the second metal plate 20 . the metal gasket of the fourth embodiment is suitable for the case in which a compressive transformation quantity due to the bead is small as compared to the metal gasket of the first embodiment , thereby saving materials and reducing the weight and size . also , the thickness of the gasket can be reduced . as shown in fig1 , 14 , the metal gasket according to the fifth embodiment differs from the metal gasket according to the fourth embodiment , because the middle plate 40 and the surface pressure assistance plate 30 are formed by one sheet of surface pressure assistance plate 30 . the other structure of the metal gasket of the fifth embodiment is the same as that of the metal gasket of the fourth embodiment . incidentally , in fig1 , the depressions and the projections of the surface pressure assistance plate 30 are made to be flat on the inner perimeter side , and have curved projections only on one side of the half bead 11 side on the outer perimeter side . also , in fig2 - 24 , in the metal gasket according to the first - fifth embodiments , instead of disposing the surface pressure assistance plate 30 inside between the sloping part 11 b ( 21 b ) of the half bead 11 ( 21 ) and the flat part 11 c ( 21 c ) on the outer perimeter side , the surface pressure assistance plate 30 is disposed inside the flat part 11 c ( 21 c ) on the outer perimeter side of the half bead 11 ( 21 ). in this case , the surface pressure moderation effect on the corner part of the half bead 11 ( 21 ) is a little . however , the elasticity of the half bead 11 ( 21 ) can be enhanced by the elasticity of the surface pressure assistance plate 30 , so that a creep relaxation of the half bead 11 ( 21 ) can be prevented . according to the metal gasket 1 with the above - mentioned structure , the surface pressure assistance plate 30 forming the depressions and the projections is disposed with respect to the thickness direction , so that a creep relaxation of the half bead can be prevented by using the elastic effect due to the depressions and the projections of the surface pressure assistance plate 30 . also , the surface pressure generated in the half bead 11 ( 21 ), especially , a force generated in the corner part of the half bead 11 ( 21 ) can be eased , so that the member clamping the metal gasket 1 can be prevented from being scratched , thereby securing an excellent seal quality . the disclosure of japanese patent application no . 2006 - 136113 , filed on may 16 , 2006 , is incorporated in the application . while the invention has been explained with reference to the specific embodiments of the invention , the explanation is illustrative and the invention is limited only by the appended claims .