An arrangement comprising a member in which is provided an installation groove and a gasket which is installed to said installation groove, wherein said member is one member of two members configured to face to each other and wherein said gasket is configured to come into close contact with the other member, the gasket comprising: a seal projection (3, 4, 5, 6) which comes into contact with a groove side surface of said installation groove, the seal projection (3, 4, 5, 6) being provided over a whole periphery of said gasket in a side surface of said gasket, wherein a concavity is provided in a part on a circumference of said gasket in the side surface of said gasket, the concavity being provided for venting air when said gasket is installed to said installation groove, wherein said seal projection (3, 4, 5, 6) is provided on each of an inner peripheral surface and an outer peripheral surface of said gasket, and said concavity is provided on each of the inner peripheral surface and the outer peripheral surface of said gasket in correspondence to the seal projection (3, 4, 5, 6), characterized in that a plurality of said seal projections (3, 4, 5, 6) are provided on the inner peripheral surface and the outer peripheral surface of said gasket and are formed into mutually facing trough portions between the adjacent seal projections (3, 4, 5, 6), and wherein said concavity is provided in such a manner as to satisfy a relationship w 1 ≥ w 0 > w 1 ', and w 0 > w 2 > w 2 ', in which w 1 is a width between a leading end portion of the inner peripheral side seal projection and a leading end portion of the outer peripheral seal projection at a gasket circumferential position where said concavity is not provided, w 2 is a width between a root portion of the inner peripheral side trough portion and a root portion of the outer peripheral side trough portion, w 1 ' is a width between the leading end portion of the inner peripheral side seal projection and the leading end portion of the outer peripheral side seal projection at a gasket circumferential position where said concavity is provided, w 2 ' is a width between the root portion of the inner peripheral side trough portion and the root portion of the outer peripheral side trough portion, and w 0 is a groove width of said installation groove. a seal projection (3, 4, 5, 6) which comes into contact with a groove side surface of said installation groove, the seal projection (3, 4, 5, 6) being provided over a whole periphery of said gasket in a side surface of said gasket, wherein a concavity is provided in a part on a circumference of said gasket in the side surface of said gasket, the concavity being provided for venting air when said gasket is installed to said installation groove, wherein said seal projection (3, 4, 5, 6) is provided on each of an inner peripheral surface and an outer peripheral surface of said gasket, and said concavity is provided on each of the inner peripheral surface and the outer peripheral surface of said gasket in correspondence to the seal projection (3, 4, 5, 6), characterized in that a plurality of said seal projections (3, 4, 5, 6) are provided on the inner peripheral surface and the outer peripheral surface of said gasket and are formed into mutually facing trough portions between the adjacent seal projections (3, 4, 5, 6), and wherein said concavity is provided in such a manner as to satisfy a relationship w 1 ≥ w 0 > w 1 ', and w 0 > w 2 > w 2 ', in which w 1 is a width between a leading end portion of the inner peripheral side seal projection and a leading end portion of the outer peripheral seal projection at a gasket circumferential position where said concavity is not provided, w 2 is a width between a root portion of the inner peripheral side trough portion and a root portion of the outer peripheral side trough portion, w 1 ' is a width between the leading end portion of the inner peripheral side seal projection and the leading end portion of the outer peripheral side seal projection at a gasket circumferential position where said concavity is provided, w 2 ' is a width between the root portion of the inner peripheral side trough portion and the root portion of the outer peripheral side trough portion, and w 0 is a groove width of said installation groove.