To clean even such a part as having parts to be cleaned and not cleaned with cleaning liquid efficiently in safety without dismantling the parts.

Protective tube 16 of a thermocouple 11 is inserted into a cleaning tank 1 through an insertion opening 1b without dismantling thermocouple 11 and held in place while sealing the insertion opening 1b hermetically by pushing an O-ring 7. Acid is then fed from a storage tank 2 into a cleaning tank 1 and a film adhering to the outer circumferential surface of the protective tube 16 is removed by etching with acid. Pure water is then fed to the cleaning tank 1 and the acid adhering to the protective tube 16 is washed away. Top of the cleaning tank 1 is partitioned by the upper wall 1a and the insertion opening 1b is sealed with the O-ring 7 thus blocking flow-out of acid vapor to the outside of the cleaning tank 1. Furthermore, leakage of acid vapor from the insertion opening 1b is blocked by forming an air flow in the upper space of the cleaning tank 1 from an air inlet 1d at the upper part of the cleaning tank 1 to the exhaust opening 1e.