PURPOSE: To permit to transfer heat effectively even when the transfer of heat is effected without employing a circulating pump while even pressure losses in a pipeline and heat radiator are designed to be big enough by a method wherein heat medium is sent out by utilizing vapor pressure and is re-circulated by utilizing vacuum effect.

CONSTITUTION: When a heating source 14 is operated, a part of the liquid phase heat medium, poured into a heater 13, becomes saturated vapor, the heat medium in the heater 13 is sent into a heat radiating coil 17 by the pressure of the vapor through a conduit 15 and is reserved into a liquid reserving tank 19 after providing ambient air with the heat thereof. When the level of liquid in the heater 13 is detected by a low liquid level sensor 20 and the operation of the heating source 14 is stopped, the heater 13 is cooled and vacuum is generated in the heater 13 due to the condensation of the vapor existing in the heater 13. When the inside of the heater 13 has become vacuum, the heat medium in the liquid reserving tank 19 recirculates into the heater 13 from a discharging pipe 18 through the radiating coil 17 and the conduit 15. When the liquid level in the heater 13 has become higher than a predetermined level, the heating source 14 is put ON and the heating is started again.