To provide a means to prevent backflow of drain in a catch basin.

An upstream pipe connection part 2 and a downstream pipe connection part 12 are provided at a lower part of a catch basin. In this case, the upstream pipe connection part 2 comprises a pipe main body 23 and a socket part 21 formed at an end part of the pipe main body 23. The pipe main body 23 is formed in a downward larger diameter than the socket part 21 to form a step part 22 at the boundary between the pipe main body 23 and the socket part 21, a valve element 3 is suspended from an inner upper part of the upstream pipe connection part 2 to be capable of oscillation, and the valve element 3 oscillates by buoyancy to be brought into contact with the step part 22 as a water level rises. A conduit of the upstream pipe connection part 2 is thus closed.