To facilitate an operation to perform maintenance services such as the replacement of a filter in a filter housing and at the same time, reduce the overall height of the filter housing.

This filter housing 22 is equipped with a suction pipe 15 and a discharge pipe 16 and a filter 23 is stored by a fixed housing 17 retaining the filter 23 and a top housing 19 to be mounted on the top of the fixed housing 17 in a freely detachable manner and in such a way that the top housing 19 is brought into close contact with the fixed housing 17. In addition, an air vent pipe 27 is provided which is connected to an air vent valve 29 provided on the fixed housing 17 side and runs through the fixed housing 17 extending upward. The upper end of the air vent pipe 27 is positioned near the inner top part of the top housing 19.