Abstract: A cap of a fuel tank comprising a lid member constructed of upper and lower plates to form a passage communicated with the air between both plates, a seating plate underlying said lid member and provided with a plurality of bores circularly arranged in the peripheral portion of the central flat field thereof, a diaphragm of a flexible material underlying said seating plate and provided with a perforation in the central portion thereof and an annular ridge surrounding the perforation on the upside thereof, a valve plate underlying said diaphragm and provided with a perforation in register with the perforation of said diaphragm, a casing disposed at the lowest position in the cap and provided with a perforation in the bottom thereof, the peripheral portion of said diaphragm being fixedly interposed between the peripheral portion or said lower plate with intermediation of said seating plate and the flange portion of said casing, and a coil spring held in said casing and adapted to urge said valve plate upwardly.