PURPOSE: To prevent the formation of ice in an internal operation part of an air valving mechanism and to easily remove and replace parts.

CONSTITUTION: A pair of plates 10, 12 formed in the opposite cavity recessed parts are fastened by a bolt 18, a cavity 112 is formed therebetween, and a cylinder 46, a bush 30 and a block 38 are fit into the cavity. The cavity 112 is ventilated to the atmospheric air through a muffler connected thereto through a panel 108. Whereby the driving air is exhausted not through a perpendicular air passage, so that the formation of ice on a valve element can be prevented. The mechanism is modular in nature, and a tool for deassembling is scarcely needed, which makes the maintenance and repairment easily.