To restrain vibrations and noises from being generated by a drainage flow in a leg joint on the lowermost floor.

An inside diameter of a straight pipe section 21 is set to become gradually larger to a boundary between the straight pipe section and a curved section 22 from a position (an upper end 23 of the straight pipe section) to be connected to an assembled joint 31 for drainage, connected to the straight pipe section 21. Thus, since a water film of drainage flowing down becomes uniformly thinner along the circumferential direction of the inner surface of the pipe on the inner surface of the straight pipe section 21 of the leg joint 20, an impact on a bottom surface of the curved section 22 by the fall of water becomes smaller than ever for the alleviation of the vibrations and the decrease of a noise level. Additionally, since the water film becomes uniformly thinner along the circumferential direction of the inner surface of the pipe, a ventilation core, passing through the center of the pipe, of the leg joint 20 is also expanded in a stable shape (an approximately circular shape). Consequently, unnecessary pressure fluctuations are hardly caused so that the crushing of a trap can be prevented.