An air conditioner (1; 101), comprising an indoor unit, an outdoor unit, and a refrigerant circuit (10; 110) connecting the indoor unit with the outdoor unit, wherein the indoor unit has therein an indoor heat exchanger provided to oppose to a fan and a radiation panel provided on a surface of the indoor unit, and the outdoor unit comprises a decompression structure, an outdoor heat exchanger, and a compressor, wherein the air conditioner (1; 101) is configured to perform a heating operation, and the refrigerant circuit (10; 110) includes: (a) a principal channel in which the decompression structure, the outdoor heat exchanger, and the compressor are provided in this order from an upstream towards a downstream side during the heating operation; a first channel, in which the indoor heat exchanger is provided, the first channel connecting a branching section provided downstream of the compressor during the heating operation with a merging section provided upstream of the decompression structure during the heating operation; a second channel provided with the radiation panel, which connects the branching section and the merging section with the first channel in parallel; and a valve structure configured to perform switching over between a state where a refrigerant flows in the radiation panel and a state where the refrigerant does not flow in the radiation panel; or (b) a principal channel connecting the indoor heat exchanger with the radiation panel; a branching channel having ends connected respectively to a branching section which is located between the indoor heat exchanger and the radiation panel and a branching section which is located on the opposite side to the branching section with respect to the radiation panel; and a valve structure configured to perform switching over between a state where the refrigerant flows in the radiation panel but does not flow in the branching channel and a state where the refrigerant flows in the branching channel but does not flow in the radiation panel, the air conditioner (1; 101) further comprising: an indoor heat exchanger temperature sensor provided to the indoor heat exchanger; and a panel temperature sensor (25; 26) provided downstream of a radiator of the radiation panel during the heating operation and between the radiator of the radiation panel and the valve structure (23; 123), or provided upstream of the radiator of the radiation panel; characterized by further comprising: a defect detector configured to detect occurrence of a defect in the valve structure (23, 123) based on a temperature detected by the panel temperature sensor (25; 26) and a temperature detected by the indoor heat exchanger temperature sensor. (a) a principal channel in which the decompression structure, the outdoor heat exchanger, and the compressor are provided in this order from an upstream towards a downstream side during the heating operation; a first channel, in which the indoor heat exchanger is provided, the first channel connecting a branching section provided downstream of the compressor during the heating operation with a merging section provided upstream of the decompression structure during the heating operation; a second channel provided with the radiation panel, which connects the branching section and the merging section with the first channel in parallel; and a valve structure configured to perform switching over between a state where a refrigerant flows in the radiation panel and a state where the refrigerant does not flow in the radiation panel; or (b) a principal channel connecting the indoor heat exchanger with the radiation panel; a branching channel having ends connected respectively to a branching section which is located between the indoor heat exchanger and the radiation panel and a branching section which is located on the opposite side to the branching section with respect to the radiation panel; and a valve structure configured to perform switching over between a state where the refrigerant flows in the radiation panel but does not flow in the branching channel and a state where the refrigerant flows in the branching channel but does not flow in the radiation panel, an indoor heat exchanger temperature sensor provided to the indoor heat exchanger; and a panel temperature sensor (25; 26) provided downstream of a radiator of the radiation panel during the heating operation and between the radiator of the radiation panel and the valve structure (23; 123), or provided upstream of the radiator of the radiation panel; characterized by further comprising: a defect detector configured to detect occurrence of a defect in the valve structure (23, 123) based on a temperature detected by the panel temperature sensor (25; 26) and a temperature detected by the indoor heat exchanger temperature sensor.