A lock member capable of contacting an engaging projection of an inserted pipe section from a direction of the pipe axis and relatively rotatably attachable between an annular wall portion of a receiving pipe section and the engaging projection of the inserted pipe section includes engaging stopper portions projecting therefrom for contacting an inward surface of the annular wall portion from the direction of the pipe axis. The annular wall portion has attachment/detachment recesses formed therein for allowing inserting and removing movements of the engaging stopper portions in the direction of the pipe axis. An elastic urging member is provided between an extending portion projecting from the lock member to the outside of the pipe through an inner peripheral surface of the annular wall portion and the outer peripheral surface of the inserted pipe section, and an outward surface of the annular wall portion.