Pressure cap

A cap body 3 of a synthetic resin includes a cylindrical portion 7 having an external thread 10 formed on its outer peripheral surface, the cylindrical portion 7 having at its lower end a shoulder 11 or an annular groove 12 for receiving an O-ring. The cylindrical portion has on its inner peripheral surface an annular, internally flanged valve stopper 13 which provides a support for a locking rib 17 of a pressure valve 6. The pressure valve 6 consists of deformation slits 16 formed along the outer periphery of a pot-shaped valve body 14, and a seal plate 15 disposed on the bottom surface of the valve body 14.