To mount or dismount a perforated plate on/from a drain port such as a drain trap easily without removing an elbow or a water drain hose and to simplify the work for taking care of and assembling in the perforated plate and the drain trap to simplify daily handling work.

This disc-like flat perforated plate 1 arranged detachably in a flat perforated plate receiver B fitted into the drain port A in the drain trap is provided with a mounting frame 2 fitted into the flat perforated plate receiver B at the outer periphery and a porous face body 3 having many water flow holes on an inner face of the mounting frame 2 and forms an elbow end connection 4 in which an elbow type hose joint D formed into substantially an L shape to connect the water drain hose C is inserted in a central part of the porous face body 3. This flat perforated plate 1 is provided with an opening part 5 communicating with the elbow end connection 4 formed in the flat perforated plate 1 to allow drainage to pass through the elbow type hose joint D or the water drain hose C, and a shutter 6 made of synthetic rubber elastic body and composed of the porous face body having the water flow holes is openably and closably arranged in the opening part 5.