There is disclosed a vehicle air conditioner of a heat pump system in which there is prevented or inhibited frosting to an outdoor heat exchanger when heating in a vehicle interior is beforehand performed during plug-in, thereby realizing comfortable heating in the vehicle interior during running and also extending a running distance. The vehicle air conditioner includes a heating medium circulating circuit 23 to heat air to be supplied from an air flow passage 3 to a vehicle interior, a controller has frosting estimation means for estimating frosting to an outdoor heat exchanger 7, and when a heating mode is executed in a state where a power is supplied from an external power source to a compressor 2 or a battery which supplies the power to drive the compressor 2, the controller executes the heating by a heating medium circulating circuit 23, in a case where the frosting to the outdoor heat exchanger 7 is predicted on the basis of the estimation of the frosting estimation means.