An air conditioner for a vehicle includes a vapor compression refrigeration cycle switchable between a heat pump cycle and a cooler cycle, a heat core configured to heat air to be blown into a vehicle compartment by using coolant of an engine of the vehicle as a heat source, and a controller configured to control operation of the vapor compression refrigeration cycle. The controller controls the vapor compression refrigeration cycle to be operated as the cooler cycle so as to perform a defrosting control of the outdoor heat exchanger, and outputs an operation request signal to the engine, when the controller determines that the outdoor heat exchanger is frosted.