To provide a lock structure improving workability, facilitating assembly, decreasing the number of parts, and making excellent in also economical efficiency.

In a lock structure fixing an insert fitting member to an insert fitted member fitted with this insert fitting member by a snap ring 13, in the periphery of the insert fitting member or insert fitted member, a contact surface (a) into contact with the snap ring and a lock part (b) spaced with the contact surface to have a diameter slightly larger than an internal diameter of the snap ring are formed, between the contact surface and the lock part, the snap ring is positioned.