An RPM of a compressor is sensed by an RPM meter, and a sound velocity of an inlet gas sucked into the compressor is sensed by a sound velocity meter. A corrected RPM computation unit finds a corrected RPM of the compressor using a reference state quantity determined by a reference specific heat ratio, a reference gas constant, and a reference temperature of the inlet gas; the RPM sensed by the RPM meter, and the sound velocity sensed by the sound velocity meter.