To provide a novel thermosiphon cycle system which can circulate a heat medium always stably and efficiently in the case where a liquid head difference formed between a condenser and an evaporator is not always sufficient or where it is difficult to provide the condenser and the evaporator closely.

A closed cycle which comprises a condenser such as cascade condenser 14, an evaporator 19, a liquid feeding pipe 20 which feeds a liquid heat medium from the condenser side to the evaporator side and a gas returning pipe 21 which sends a gaseous heat medium from the evaporator side to the condenser side. In this cycle, the cascade condenser 14 (condenser) is installed in a position higher than that of the evaporator 19 to mainly aim at the circulation of the heat medium by a liquid head difference formed between the condenser and the evaporator. A liquid separator 22 which can control a liquid surface is provided in the vicinity of the evaporator 19.