To increase detergency at the time when various filters are subjected to ultrasonic cleaning, and to simultaneously efficiently clean the same.

In the ultrasonic cleaning method of a filer, a filter F is thrown into a cleaning tank 1 from which a cleaning liquid is 1 drained, the inside of the cleaning tank 1 is evacuated to a negative pressure (gauge pressure) of ≤0.09 Mpa, and further, a deaerated cleaning liquid (having a dissolved oxygen content of ≤3 mg/l) is introduced into the cleaning tank 1. Thereafter, the inside of the cleaning tank 1 returned to an atmospheric pressure state, the cleaning liquid in the cleaning tank 1 is circulated while being made to flow to the reverse cleaning direction to the filtering face of the filter F to remove gas in the cleaning liquid, and simultaneously, ultrasonic waves are radiated into the liquid, so that ultrasonic cleaning is performed.