PURPOSE: To provide an air conditioner of hot water flow rate control system for an automobile by which a blowoff air temperature can be controlled with high accuracy.

CONSTITUTION: After a data map to show the relationship between a hot water flow rate, engine rotating speed and the duty ratio of an ON-OFF signal is stored in a ROM 31, a necessary hot water flow rate is found from a target blowoff temperature calculated by signals from various sensors 25 to 28 and a temperature setting unit 29 by an automatic amplifier 24, and operation is performed on the duty ratio of the ON-OFF signal on the basis of the found hot water flow rate and the engine rotating speed by referring to the data map, and a result is outputted to a duty control circuit 30.