A sealing structure comprising: two members (20, 30; 40, 50) at least either of which is provided with an attaching groove (31; 41, 51) whose surface is a casting surface; and a gasket made of a rubber-like elastic body attached to the attaching groove (31; 41, 51) to seal a gap between these two members (20, 30; 40, 50), wherein the gasket includes a first sealing surface coming into surface contact with a surface of a first member out of the two members (20, 30; 40, 50); a second sealing surface coming into surface contact with a surface of a second member out of the two members (20, 30; 40, 50); and a third sealing surface coming into surface contact with a side surface of the attaching groove (31; 41, 51) on an opposite side of a side on which sealing fluid pressure is generated, and wherein said side on which sealing fluid pressure is generated is an inner circumferential side of the attaching groove (31; 41, 51) which is an annular groove; said third sealing surfaces is an outer circumferential surface of said gasket which is an annular gasket; and the gasket includes a pressure receiving groove opened to a side receiving the sealing fluid pressure which is an opposite side of the third sealing surface and provided so as to be parallel to the first sealing surfaces and the second sealing surface, characterized in that the first sealing surfaces, the second sealing surface, and the third sealing surfaces are formed in equal shapes to those of surfaces of opposed members which these sealing surfaces come into surface contact with in a state before the gasket receives an external force, respectively, and the side of the gasket receiving the sealing fluid pressure is provided with a plurality of projecting portions (12, 14) coming into close contact with a side surface of the attaching groove (31; 41, 51) on the side on which the sealing fluid pressure is generated on both sides with the pressure receiving groove in-between, for restricting coming off of the gasket from the attaching grooves (31; 41; 51) at the time of assembling. two members (20, 30; 40, 50) at least either of which is provided with an attaching groove (31; 41, 51) whose surface is a casting surface; and a gasket made of a rubber-like elastic body attached to the attaching groove (31; 41, 51) to seal a gap between these two members (20, 30; 40, 50), wherein the gasket includes a first sealing surface coming into surface contact with a surface of a first member out of the two members (20, 30; 40, 50); a second sealing surface coming into surface contact with a surface of a second member out of the two members (20, 30; 40, 50); and a third sealing surface coming into surface contact with a side surface of the attaching groove (31; 41, 51) on an opposite side of a side on which sealing fluid pressure is generated, and wherein said side on which sealing fluid pressure is generated is an inner circumferential side of the attaching groove (31; 41, 51) which is an annular groove; said third sealing surfaces is an outer circumferential surface of said gasket which is an annular gasket; and the gasket includes a pressure receiving groove opened to a side receiving the sealing fluid pressure which is an opposite side of the third sealing surface and provided so as to be parallel to the first sealing surfaces and the second sealing surface,