PURPOSE: To contrive improving deaeration effect of washing liquid and improving washing effect by making the deaerated washing liquid by a boiling tank form forming a membrane of liquid temperature which is normally near boiling temperature in the upper end part of a washing tank and making the washing area in the washing tank be at lower temperature.

CONSTITUTION: Washing liquid 7 deaerated by a boiling tank 3 is caused to flow in an upper end part 15 of a washing tank 2 to form a high temperature zone there. Work is immersed in washing liquid in the washing tank 2 to house the washing liquid 7 overflowing the high temperature zone in a recovery tank 7, and also it is circulated in the washing tank 2 through a circulating pipe 18. Furthermore, in order to feed the washing liquid 7 in the washing tank 2 to an evaporating tank 4, a recovery pipe 25 is arranged. And the arrangement position of the circulating pipe 18 is a low temperature zone of the washing tank lower part 19, and the fitting position of the recovery pipe 25 is the work immersion position which is an about, intermediate position between the washing tank upper end part 15 high temperature zone and the washing tank lower part 19 low temperature zone. As a result, the deaeration efficiency of the washing liquid is improved and also the washing efficiency is improved.