There is disclosed an air-conditioning device of a so-called heat pump system which acquires comfortable heating of a vehicle interior by preventing or inhibiting frost formation to an outdoor heat exchanger. A controller calculates a maximum heating capability predicted value without frosting QmaxNfst as a target value of a maximum heating capability which can be generated by a radiator 4 in a range in which an outdoor heat exchanger 7 is not frosted, and controls heating by the radiator 4 and heating by a heating medium-air heat exchanger 40 of a heating medium circulating circuit 23 on the basis of the maximum heating capability predicted value without frosting QmaxNfst and a required heating capability Qtgt which is the heating capability required for the radiator 4 to achieve the required heating capability Qtgt without causing frost formation to the outdoor heat exchanger 7.