A vehicle air conditioner comprising: a heat pump device including a compressor which compresses a refrigerant, a first interior heat exchanger provided inside a vehicle cabin, a first expansion valve, a second interior heat exchanger provided upstream of the first interior heat exchanger in an airflow direction inside the vehicle cabin, a second expansion valve, and an exterior heat exchanger provided outside the vehicle cabin, the heat pump device being formed by connecting together the compressor, the first interior heat exchanger, the first expansion valve, the second interior heat exchanger, the second expansion valve, and the exterior heat exchanger in this order via refrigerant piping; an interior air-conditioning unit which houses the first and second interior heat exchangers, which includes a blower that blows air-conditioning air toward the first and second interior heat exchangers, and which is configured to produce air-conditioned air and supply the air-conditioned air into the vehicle cabin; a frosting state detecting means for detecting a frosting state of the exterior heat exchanger; and an air-conditioning controller which controls the heat pump device and the interior air-conditioning unit, wherein the heat pump device is switched by the air-conditioning controller into any of a plurality of operation modes including: a first heating operation mode in which a refrigerant discharged from the compressor is allowed to flow through the first and second interior heat exchangers (31, 32) without being expanded by the first expansion valve and then flow through the exterior heat exchanger while being expanded by the second expansion valve; a second heating operation mode in which the refrigerant discharged from the compressor is allowed to flow through the first interior heat exchanger, bypasses the first expansion valve and the second interior heat exchanger, and then flows through the exterior heat exchanger while being expanded by the second expansion valve; and a defrosting operation mode in which the refrigerant discharged from the compressor is allowed to flow through the first interior heat exchanger, be bypassed around the first expansion valve and the second interior heat exchanger, and then flow through the exterior heat exchanger without being expanded by the second expansion valve, and the air-conditioning controller is configured to switch the operation modes of the heat pump device into the second heating operation mode and then into the defrosting operation mode if the frosting state detecting means has sensed, while the operation mode of the heat pump device is the first heating operation mode, that the exterior heat exchanger is frosted. a heat pump device including a compressor which compresses a refrigerant, a first interior heat exchanger provided inside a vehicle cabin, a first expansion valve, a second interior heat exchanger provided upstream of the first interior heat exchanger in an airflow direction inside the vehicle cabin, a second expansion valve, and an exterior heat exchanger provided outside the vehicle cabin, the heat pump device being formed by connecting together the compressor, the first interior heat exchanger, the first expansion valve, the second interior heat exchanger, the second expansion valve, and the exterior heat exchanger in this order via refrigerant piping; an interior air-conditioning unit which houses the first and second interior heat exchangers, which includes a blower that blows air-conditioning air toward the first and second interior heat exchangers, and which is configured to produce air-conditioned air and supply the air-conditioned air into the vehicle cabin; a frosting state detecting means for detecting a frosting state of the exterior heat exchanger; and an air-conditioning controller which controls the heat pump device and the interior air-conditioning unit, wherein the heat pump device is switched by the air-conditioning controller into any of a plurality of operation modes including: a first heating operation mode in which a refrigerant discharged from the compressor is allowed to flow through the first and second interior heat exchangers (31, 32) without being expanded by the first expansion valve and then flow through the exterior heat exchanger while being expanded by the second expansion valve; a second heating operation mode in which the refrigerant discharged from the compressor is allowed to flow through the first interior heat exchanger, bypasses the first expansion valve and the second interior heat exchanger, and then flows through the exterior heat exchanger while being expanded by the second expansion valve; and a defrosting operation mode in which the refrigerant discharged from the compressor is allowed to flow through the first interior heat exchanger, be bypassed around the first expansion valve and the second interior heat exchanger, and then flow through the exterior heat exchanger without being expanded by the second expansion valve, and the air-conditioning controller is configured to switch the operation modes of the heat pump device into the second heating operation mode and then into the defrosting operation mode if the frosting state detecting means has sensed, while the operation mode of the heat pump device is the first heating operation mode, that the exterior heat exchanger is frosted.