Abstract: An orifice assembly is detachably attached to a cover serving as a wall surface of an atmospheric chamber. The orifice assembly includes two inner and outer communication ports in a housing thereof. The two inner and outer communication ports are shut out by a partition wall through which an orifice having a small diameter and a communication hole having a diameter larger than that of said orifice, and a check valve is disposed in the communication hole to close the latter therewith. One of the two inner and outer communication ports is communicated with an atmospheric chamber and the other inner and outer communication ports is communicated with the atmosphere. When the pressure in the atmospheric chamber becomes higher than the atmospheric pressure, the communication hole is closed with the check valve, and when the pressure in the atmospheric chamber 48 is equal to or becomes lower than the atmospheric pressure, the communication hole is opened.