A receptacle of a bodily fluid trapping unit is defined by first and second sheets bonded together along respective peripheral edges thereof. The first sheet is formed with openings extending therethrough in its thickness direction. Backflow preventing means includes a flat valve disc provided on the side of the first sheet facing the receptacle, a sheet-like support base serving to prevent the valve disc from falling into the receptacle and a spacer interposed between the first sheet and the support base. A peripheral edge of the valve discs upper surface is adapted to come in water-tight contact with the associated valve seat formed on the side of the openings peripheral edge facing the receptacle. Between the first sheet and the support base, a space is defined by a spacers opening so that the valve disc is movably supported in the thickness direction of the spacer.
