To solve the problem of the necessity of shortening an interval of cleaning to prevent clogging caused when oil and dirt easily stick to an inner wall since there is no flow speed when an inner diameter of a valve is small, in addition to the problem of being difficult to drain floated oil when water of gathered drain water and the oil separate when there is naturally no force in waste water since a float body does not rise by buoyancy when a flow rate is small, and a problem that a valve frequently assumes such a partially and constantly opened state that the valve is opened and then closed, and vice versa in continuous flowing since opening-closing of a valve seat for opening the valve is mainly performed with a dead weight of a float and the buoyancy of the float body in the first place.

The drain water 101 gathered in cases 10 and 20 is instantly and suddenly increased so that the valve 18 is instantly, completely and surely opened.