PURPOSE: To shorten a drop-in time for a liquid refrigerant, to shorten a switching period, and to improve a capacity of heat conveying amount by providing an annular circulation passage in which a gas/liquid separator, a heat radiator, a second check valve and a liquid receiver are sequentially connected, and a liquid storage unit in which an upper part is opened and a lower part is closed in the receiver.

CONSTITUTION: Liquid refrigerant of a low temperature is stored in a liquid receiver 5 even at the time of opening a switching valve 8, a temperature of a wall 21 in the receiver 5 is lowered to form a lead of starting to condense gas refrigerant. A pressure reduction in the receiver 5 occurs due to condensation of the gas refrigerant with subcooled liquid refrigerant of this liquid reservoir 20 without pressure reduction start delay time, and the liquid refrigerant is simultaneously sucked into the receiver 5 upon closing of the valve 8. Thus, the pressure reduction start delay time is eliminated thereby to shorten a closing time of the valve 8 thereby to reduce a switching period, sucking-dropping-in times of the receiver 5 per unit time are increased to increase a refrigerant circulating amount, and refrigerant heating amount is increased thereby to obtain a large capacity of heat conveying amount.