PURPOSE: To raise the response and durability of heating portion for the variations in the amounts of circulating refrigerant in an air conditioner with a refrigerant-heating unit by using a system in which the output of the refrigerant heating unit is controlled on temperature rise gradients at the specific temperature of gas refrigerant coming out of the heating unit.

CONSTITUTION: When a refrigerant circulates in a heating circuit with a heating unit during the heating period, the temperature of gas refrigerant coming out of the heating unit 12 is detected 22 and put in a controller 23. In the controller 23, a fuel control portion is valve-controlled toward the increasing direction of fuel if the temperature of the gas refrigerant is below a set temperature th1 rise gradients at the time when the set temperature th1 just before the upper limit is reached is detected and the amount of fuel to be supplied in the fuel control portion is controlled. By this, the temperature of gas refrigerant coming out of the heating unit can be properly controlled just before the upper limit value, response to the variations in the amounts of circulating refrigerant can be raised, and the overheating of the refrigerant can be prevented.