PROBLEM TO BE SOLVED: To mix a gas into a cleaning liquid with high efficiency, to dissolve a gas in a cleaning liquid stably and sufficiently and thus to execute steady cleaning by introducing a gas mixed cleaning liquid into a tank to form a gas dissolved cleaning liquid and then injecting this gas dissolved cleaning liquid against a metallic strip for cleaning. SOLUTION: With regard to the gas mixed cleaning liquid introduced into the tank, for example, an ejector or the like is provided as a gas-liquid mixer in a cleaning liquid supplying tube at the tank inlet side, and simultaneously with the supply of the cleaning liquid air is supplied as a gas. Thereby, the contact area of the gas and the cleaning liquid is increased enormously, and a gas mixed cleaning liquid is prepared with a great deal of mixed gas in the cleaning liquid in a short time and is introduced into the tank. In this way, by introducing thus prepared gas mixed cleaning liquid into the tank mixing action generates in the tank, and the mixed gas in the gas mixed cleaning liquid is dissolved in the cleaning liquid sufficiently and rigidly. Resultantly, a satisfactory cleaning effect is obtained.