PURPOSE: To eliminate a water supply piping, by providing a filter at a water intake opening for a filling water, and providing a select channel for selectively connecting the water intake opening to a suction side or a discharge side of a compressor.

CONSTITUTION: A water intake opening 5 of a blower 1 is provided with a filter, and a three-way valve 13 is provided in a water intake pipe 4. The water intake opening 5 is selectively connected to a suction side or a discharge side of the blower 1 by way of a select channel formed by either of the water intake pipe 4 or a piping consisting of a pipe section upstream of a branch point between an air feeding pipe 9 and a branched discharge pipe 14, the branched discharge pipe 14 and a pipe section of the water intake pipe 4 downstream of the three-way valve 13. Accordingly, it is not necessary to provide an independent water supply pipe.