PURPOSE: To decrease sewage flowing backward by providing an uneven step section to the bottom of a main drainpipe channel so an outlet side becomes lower than an inlet side.

CONSTITUTION: A weir levee having a weir on the upper edge is projected to the bottom of a valve main body 25, and a main drainpipe channel 27 having an inlet side socket 25c and an outlet side socket 25b of a sewer with small caliber opened in the side of the valve main body is formed in one side across the weir levee. An uneven step section 28 is provided closer to the inlet socket 25c side of the main drainpipe channel 27 than the weir so that the outlet side socket 25b is lower than the inlet side socket 25c in the bottom of the main drainpipe channel 27. Sewage overflowing from one end of a U-shaped trap flows into the main drainpipe channel through the weir to run against a side wall forming the main drainpipe channel. Sewage flowing toward the inlet side of the sewage running against the side wall is made to change the stream by the uneven step section and flows toward the outlet side. Accordingly, as the step of the uneven step section is made greater, the sewage flowing backward toward the inlet side is controlled more greatly to facilitate a quick drainage.