A seal structure for connection sections, which facilitates installation and removal of a seal member from the connection sections. The seal member is placed in the connection sections between a first part and a second part. A first groove-ridge section is formed at the connection section of the first part, and a second groove-ridge section is formed at the connection section of the second part. The seal member has, on one face, a first fitting section fitting to the first groove-ridge section, and also has, on the other face, a second fitting section fitting to the second groove-ridge section. Press-fitting interferences in the wall-thickness direction are provided at the first and second fitting sections. The seal member has an extended section protruding radially outward from an annular body section where the first and second fitting sections are provided. The extended section protrudes to a position outside the connection sections of the first and second parts.