To smoothly lead dew condensation attached to a lower surface of a top wall to a center portion of the top wall, and return the condensed water to a waste stack by tapering the lower surface of the top wall downward to its center portion.

A surface under a top wall 16 is tapered to its center to show a recessed form. Dew condensation attached to a lower surface of a lower surface composing member 16B is dropped to the center portion of the lower surface, and returned to a waste stack 30 connected to a pressure control valve 1. The drew condensation attached to an inner surface of a circular side wall 15 is stored in a water reservoir groove 14, while its amount is extremely small for enabling natural evaporation. Therefore, the condensed water does not over flow from prescribed capacity of water reservoir groove 14. The dew condensation on an upper surface of a circular valve element 19 is returned into an inside circular body 5. Condensed water entering a portion between a top wall main body 16A and the lower surface composing member 16B is returned to the waste stack 30 connected to the pressure control valve 1 through a drainage hole 25.