To make a rapid correction at the time of liquid level being fluctuated so as to improve stability of the liquid level and liquid level detecting accuracy by providing a pressure equalizing fine for communicating both gas phase parts of an evaporator and an overflow tank setting the liquid level.

When a heat exchanger means 2 is actuated, evaporated liquid R in a heat exchanger column 21 is put in a high temperature state and fed into an evaporator 1, and the evaporated liquid R is evaporated in the evaporator 1, so that the liquid level is lowered. Between the evaporator 1 and an overflow tank 4, a gas phase part and a liquid phase part are connected by a pressure equalizing pipe 5 and a connecting pipe 34. The liquid level of the overflow tank 4 is therefore lowered by the lowered part of the liquid level of the evaporator 1 by a siphon phenomenon so as to be balanced. Accordingly, while a liquid feeding means 3 is operated, the liquid level of the overflow tank 4 is set, so that the liquid level of the evaporator 1 and overflow tank 4 are maintained at the same level so as to improve the liquid level stability of the evaporator 1.