To prevent a filter from dropping into a suction port while ensuring an opening area required for sucking air so as to prevent lowering of the efficiency of air feed, and to reduce vibration and noise so as to greatly prolong the replacement cycle of filters.

A filter accommodating part 19 is provided on the suction port side of a pump casing 8, and is formed in the bottom surface thereof with a plurality of small holes 17 for sucking air. Filter receiving ribs 60 for supporting a filter 31 is planted to the bottom surface of the filter accommodating part 19 so that the small holes 17 are communicated with one another. The filter 31 is set on the filter receiving ribs 60.