PURPOSE: To permit efficient energy-saving operation even under the condition of low atmospheric temperature by a method wherein a combustion type room heating, suitable for heating a greenhouse for horticulture, is combined with the air-cooled heat pump type room heating.

CONSTITUTION: When the atmospheric temperature is comparatively high, the room heating is effected by the air-cooled heat pump type air conditioner without employing a kerosene warm air heater 2. When the atmospheric temperature is reduced and the room heating capacity of the air-cooled heat pump type air-conditioner is reduced, the room heating is effected by employing the kerosene warm air heater 2 in parallel. In this case, the waste gas 8 of the kerosene warm air heater 2 passes through an exhaust pipe 6 and is released to the outside of the room through an exhaust tube 6a, and the exhaust pipe 6 is being heated to a high temperature. On the other hand, an outdoor unit 4 is effecting ventilating operation for exchanging heat with the atmosphere 10, therefore, the atmosphere, induced from a suction duct 9, is heated by exchanging heat with the exhaust pipe 6 penetrating the suction duct 9. The atmosphere 10 enters into the outdoor unit 4 under heated condition, therefore, the operation of the air-cooled heat pump air conditioner under a condition that the suction atmosphere temperature is high may be effected even when the atmospheric temperature is low.