An internal combustion engine comprising: a cylindrical cylinder liner in which a piston reciprocates, a cylinder body on which the cylinder liner is fitted, a cylinder head provided to face the cylinder body and configured to close an opening of the cylinder liner, and a seal portion being a cold rolled steel plate material with a Young's modulus of 205 [GPa], provided to surround an outer circumferential side of the opening and configured to seal between the cylinder head and the cylinder liner, and a gap member which is suitable for being used in the internal combustion engine, wherein the cylinder liner includes an outer circumferential side end surface and an inner circumferential side end surface which are formed to be orthogonal to a center axis of the cylinder liner, wherein the outer circumferential side end surface is formed to include a surface of a flange portion facing the cylinder head, wherein the inner circumferential side end surface is formed_closer to the center axis of the cylinder liner than the flange portion, and is formed at a position projecting to the cylinder head side relative to the outer peripheral side end surface via a first stepped portion, wherein the gap member is disposed in a gap between the cylinder head and the cylinder liner on an inner circumferential side of the seal portion, the gap member is formed in an annular belt shape corresponding to the gap between the cylinder head and the cylinder liner, and the gap member includes an annular portion sandwiched between the cylinder head and the inner circumferential side end surface, and a curved portion integrally formed on an outer circumferential side of the annular portion and which extends along the first stepped portion from an outer circumferential side end portion of the annular portion, characterized in that the cylinder head includes an inner circumferential side opposing surface and an outer circumferential side opposing surface which face the inner circumferential side end surface and the outer circumferential side end surface of the cylinder liner, respectively, wherein the outer circumferential side opposing surface is formed at a position protruding toward the cylinder liner side of the inner circumferential side opposing surface, wherein the cylinder head includes a second stepped portion between the inner circumferential side opposing surface and the outer circumferential side opposing surface, wherein the gap member is formed of any one of polytetrafluoroethylene and a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer which is a resin material having a lower Young's modulus lower than the seal portion, and the seal portion is disposed away from the outer circumferential side of the second stepped portion. a cylindrical cylinder liner in which a piston reciprocates, a cylinder body on which the cylinder liner is fitted, a cylinder head provided to face the cylinder body and configured to close an opening of the cylinder liner, and a seal portion being a cold rolled steel plate material with a Young's modulus of 205 [GPa], provided to surround an outer circumferential side of the opening and configured to seal between the cylinder head and the cylinder liner, and a gap member which is suitable for being used in the internal combustion engine, wherein the cylinder liner includes an outer circumferential side end surface and an inner circumferential side end surface which are formed to be orthogonal to a center axis of the cylinder liner, wherein the outer circumferential side end surface is formed to include a surface of a flange portion facing the cylinder head, wherein the inner circumferential side end surface is formed_closer to the center axis of the cylinder liner than the flange portion, and is formed at a position projecting to the cylinder head side relative to the outer peripheral side end surface via a first stepped portion, wherein the gap member is disposed in a gap between the cylinder head and the cylinder liner on an inner circumferential side of the seal portion, the gap member is formed in an annular belt shape corresponding to the gap between the cylinder head and the cylinder liner, and the gap member includes an annular portion sandwiched between the cylinder head and the inner circumferential side end surface, and a curved portion integrally formed on an outer circumferential side of the annular portion and which extends along the first stepped portion from an outer circumferential side end portion of the annular portion, characterized in that the cylinder head includes an inner circumferential side opposing surface and an outer circumferential side opposing surface which face the inner circumferential side end surface and the outer circumferential side end surface of the cylinder liner, respectively, wherein the outer circumferential side opposing surface is formed at a position protruding toward the cylinder liner side of the inner circumferential side opposing surface, wherein the cylinder head includes a second stepped portion between the inner circumferential side opposing surface and the outer circumferential side opposing surface, wherein the gap member is formed of any one of polytetrafluoroethylene and a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer which is a resin material having a lower Young's modulus lower than the seal portion, and the seal portion is disposed away from the outer circumferential side of the second stepped portion.