A seal structure and a flow-path connection part including a pair of flanges arranged to face each other, the seal structure comprising: a gasket having a flat portion and a protrusion provided symmetrically on front and back sides of the gasket at an outer periphery thereof or near the outer periphery; and the flanges each having a receiving groove provided annularly in a connected surface thereof, wherein the receiving groove is wider than the protrusion and has an arc-shaped deepest portion against which a top portion of the protrusion abuts, the top portion of the protrusion has a curved surface formed such that the curvature of the top portion is greater than the curvature of the deepest portion of the receiving groove, and that a height of the protrusion is greater than a depth of the receiving groove, wherein a sectional shape of the top portion of the protrusions is semicircular or semi-elliptic; the receiving groove has a semicircular or semi-elliptic sectional shape, so that when the flanges are approached to each other in a facing state of the receiving grooves, where the gasket is fitted into the receiving grooves in not-centered relation, each protrusion undergoes a force acting in a direction toward the deepest portion, and movement of the gasket is stopped upon the protrusion reaching the deepest portion and respective centers of the gasket and the flanges being aligned with each other, and when the flanges are connected, where the flat portion are collapsed by center side ends of the flanges, the gasket is press-contacted with the flanges at the deepest portion and the center side end, while leaving a play space in the receiving grooves. a gasket having a flat portion and a protrusion provided symmetrically on front and back sides of the gasket at an outer periphery thereof or near the outer periphery; and the flanges each having a receiving groove provided annularly in a connected surface thereof, wherein the receiving groove is wider than the protrusion and has an arc-shaped deepest portion against which a top portion of the protrusion abuts, the top portion of the protrusion has a curved surface formed such that the curvature of the top portion is greater than the curvature of the deepest portion of the receiving groove, and that a height of the protrusion is greater than a depth of the receiving groove, wherein a sectional shape of the top portion of the protrusions is semicircular or semi-elliptic; the receiving groove has a semicircular or semi-elliptic sectional shape, so that when the flanges are approached to each other in a facing state of the receiving grooves, where the gasket is fitted into the receiving grooves in not-centered relation, each protrusion undergoes a force acting in a direction toward the deepest portion, and movement of the gasket is stopped upon the protrusion reaching the deepest portion and respective centers of the gasket and the flanges being aligned with each other, and when the flanges are connected, where the flat portion are collapsed by center side ends of the flanges, the gasket is press-contacted with the flanges at the deepest portion and the center side end, while leaving a play space in the receiving grooves.