PURPOSE: To prevent a large amount of storm water from flowing into sewerage through a street inlet, in a heavy rainfall.

CONSTITUTION: One end of an imperfect-valve support 4 is swingably mounted in the vicinity of an outlet 21 for fluid and an imperfect valve 5 closing the outlet 21 partially and not closing the outlet 21 perfectly is provided on the other end of the imperfect-valve support 4. The imperfect valve 5 can be reciprocated between an imperfectly closing position partially closing the outlet 21 therewith and an opening position being apart from the outlet 21, by the swing of the imperfect-valve support 4, and a float 42 is provided on the imperfect- valve support 4. When fluid level decreases, the imperfect valve 5 lowers to be in the vicinity of the opening position thereof, owing to the gravitation of the imperfect valve 5, the imperfect-valve support 4 and so on. When the fluid around them increases, the imperfect valve 5 rises to be in the vicinity of the imperfectly closing position, owing to buoyancy by the float 42.