PURPOSE: To smoothly convey heat, by stopping a heating operation of one heater when operating the other heater.

CONSTITUTION: When the density of a vapor of a heat-transmitting medium in a gaseous phase part 8 of a heater 1 is increased, the vapor flows from the heater 13 into a condenser 5 through a passage 4. The medium flows from the condenser 5 into a heater 23 through a passage 6. When a heating element 22 is connected to a power source 3 when the quantity of the refrigerant liquid contained in the heater 1 is reduced, the temperature in the heater 21 is raised, the pressures in the heater 21, the condenser 5 and the heater 1 is reduced in this order, while the heat-transmitting medium is vaporized in the heater 23, the vapor is condensed into a liquid in the condenser 5, and the liquid flows back into the heater 13. Accordingly, by alternating the heating operations of the heaters 13 and 23, the heat-transmitting medium can be continuously circulated.