PURPOSE: To provide an air conditioner which has rapid heating performance and is excellent in controllability and productivity by regulating a circulating quantity of a refrigerant to exchange heat with cooling water used to heat air for air-conditioning, and promoting a temperature rise in the cooling water used to heat the air for air-conditioning.

CONSTITUTION: When a compressor 10 is driven, saturated vapor of a refrigerant filled in a circuit in response to a starting time temperature is compressed, and pressure is increased, and a temperature is raised. When this high pressure- high temperature refrigerant gas is supplied to the liquid phase side of a liquid tank 12, since it heats and evaporates a liquid refrigerant stored there, a refrigerant gas quantity in the circuit suddenly increases. The refrigerant gas passing through a warm water radiator 16 increases in a short period, and heat exchange is promoted through the warm water radiator 16, and cooling water passing through the warm water radiator 16 is rapidly raised in the temperature. As a result, since heat exchange between the cooling water whose temperature is raised by passing through the warm water radiator 16 and air for air- conditioning is promoted in a heater core 7, the interior is rapidly heated.