A deaeration device for bubbling dissolved air in the cleaning liquid by cavitation is connected on a cleaning-liquid circulation path so as to bubble the dissolved air in the cleaning liquid flowing through the cleaning-liquid circulation path, and the bubbled dissolved air flows back to the cleaning tank together with the cleaning liquid so that the bubbled dissolved air is ejected from the liquid surface of the cleaning tank to the outside of the tank. Moreover, a propeller-type pump is used as the circulating pump, and the dissolved air concentration of the cleaning liquid is controlled within a range of 2.5 to 3.5 mg/l.