A seal structure for an internal combustion engine at a junction of three members including: an engine main body section including a cylinder block and a cylinder head; a chain casing assembled onto one end surface of a periphery of the engine main body section via a seal member (2, 22); and a cover member (5, 7) assembled via another seal member in a form extending on upper surface sides or lower surface sides of both of the engine main body section and the chain casing, the seal structure comprising: a cover side seal member arranged in a closed loop shape, which extends on upper surface sides or lower surface sides of both of the engine main body section and the chain casing, and pressed and held against the cover member (5, 7); a casing side seal member (2, 22) arranged at left or right end section of one end surface of the engine main body section onto which the chain casing is attached, and pressed and held against the chain casing; characterised in that it further includes a receiving concavity formed to extend on a junction surface of the chain casing to the engine main body section and another junction surface of the chain casing to the cover member (5, 7) and including a receiving surface parallel to the junction surface to the cover member (5, 7); and an extremely thick connection block section (13, 23) formed integrally on one end section of the casing side seal member (2, 22) in an elongate direction of the casing side seal member (2, 22), fitted into and retained by the receiving concavity (13, 23), and projected more toward the cover member side (5, 7) than the junction surface of the chain casing to the cover member (5, 7) in a free state, an end surface of the extremely thick connection block section (13, 23) opposing against the cover member (5, 7) providing a flat abutting surface (13a, 23a), and, in a state in which the three members of the engine main body section, the chain casing, and the cover member (5, 7) are joined and assembled together, the cover side seal member being contacted on the abutting surface (13a, 23a) of the connection block section (13, 23) under pressure to interconnect both of the seal members. a cover side seal member arranged in a closed loop shape, which extends on upper surface sides or lower surface sides of both of the engine main body section and the chain casing, and pressed and held against the cover member (5, 7); a casing side seal member (2, 22) arranged at left or right end section of one end surface of the engine main body section onto which the chain casing is attached, and pressed and held against the chain casing; characterised in that it further includes a receiving concavity formed to extend on a junction surface of the chain casing to the engine main body section and another junction surface of the chain casing to the cover member (5, 7) and including a receiving surface parallel to the junction surface to the cover member (5, 7); and an extremely thick connection block section (13, 23) formed integrally on one end section of the casing side seal member (2, 22) in an elongate direction of the casing side seal member (2, 22), fitted into and retained by the receiving concavity (13, 23), and projected more toward the cover member side (5, 7) than the junction surface of the chain casing to the cover member (5, 7) in a free state, an end surface of the extremely thick connection block section (13, 23) opposing against the cover member (5, 7) providing a flat abutting surface (13a, 23a), and, in a state in which the three members of the engine main body section, the chain casing, and the cover member (5, 7) are joined and assembled together, the cover side seal member being contacted on the abutting surface (13a, 23a) of the connection block section (13, 23) under pressure to interconnect both of the seal members.