PURPOSE: To provide a degassed liquid producing method and ultrasonic cleaning method and apparatus which are to produce a degassed cleaning liquid to be used for removing flashes formed at the time of a machining process or shaping cast products (metal/resin), cleaning, and carrying out sterilization, have simple structure, and which can retain a degassed liquid in degassed state for a long duration.

CONSTITUTION: A cleaning liquid L1 is prevented from being brought into contact with air by installing a protective liquid injecting system 6 to supply a protective liquid L2, which has a smaller specific gravity than the cleaning liquid L1 and is insoluble to the cleaning liquid L1, in a cleaning container 2 to store the cleaning liquid L1, mixing the protective liquid L2 with the degassed-cleaning liquid L1, and forming a film of the protective liquid L2 on the surface of the cleaning liquid L1.