To provide a drain system for building to be mainly applied to a multiple dwelling house.

This drain system comprises a falling main pipe penetrating floors, apparatus drain pipes introduced from kitchen and sanitary apparatuses in each floor, and a confluent part joint for connecting the apparatus drain pipes to the falling main pipe. The level of the confluent part joint is set to be lower than the level of a floor slab on the draining floor to provide a vertical speed component to fluid in the apparatus drain pipe for letting it to be combined with fluid in the falling main pipe. The apparatus drain pipes are disposed roughly at a constant interval around the falling main pipe to be connected to the confluent part joint. An inner wall of the confluent part joint is formed as a concave surface toward a center part.