An air conditioner, comprising: an indoor unit and an outdoor unit; the indoor unit comprising an indoor heat exchanger, a fan, a radiation heat exchanger, and a casing accommodating therein the fan and the indoor heat exchanger, wherein the casing has an outlet for blowing out air, the indoor heat exchanger is provided inside the indoor unit so as to oppose to the fan, and the radiation heat exchanger is provided on a surface of the indoor unit; the outdoor unit comprising a compressor, an outdoor heat exchanger, and a decompression structure, wherein the air conditioner comprises a refrigerant circuit having the compressor, the indoor heat exchanger, the radiation heat exchanger, the decompression structure, and the outdoor heat exchanger; wherein the air conditioner is configured to be operated in a heating operation mode for performing an indoor heating operation, the refrigerant circuit further comprising: a principal channel in which the decompression structure, the outdoor heat exchanger, and the compressor are provided in this order from an upstream side towards a downstream side during the heating operation, the principal channel extending between a branching section and a merging section; a first channel having the indoor heat exchanger, which is configured to connect the branching section on the downstream side of the compressor with the merging section provided on the upstream side of the decompression structure, during the heating operation; and a second channel having the radiation heat exchanger, which connects the branching section and the merging section in parallel with the first channel during the heating operation, a valve structure configured to adjust the amount of a refrigerant to be supplied to the radiation heat exchanger is provided to the refrigerant circuit; and wherein the air conditioner comprises a valve controller, the valve controller being configured to increase an opening degree of the valve structure after a frequency of the compressor is decreased, when a warm-air heating is performed by causing the refrigerant to flow in the indoor heat exchanger and a radiation heating is performed by causing the refrigerant to flow in the radiation heat exchanger so that a flow rate of the refrigerant flowing in the radiation heat exchanger and in the indoor heat exchanger is adjusted. an indoor unit and an outdoor unit; the indoor unit comprising an indoor heat exchanger, a fan, a radiation heat exchanger, and a casing accommodating therein the fan and the indoor heat exchanger, wherein the casing has an outlet for blowing out air, the indoor heat exchanger is provided inside the indoor unit so as to oppose to the fan, and the radiation heat exchanger is provided on a surface of the indoor unit; the outdoor unit comprising a compressor, an outdoor heat exchanger, and a decompression structure, wherein the air conditioner comprises a refrigerant circuit having the compressor, the indoor heat exchanger, the radiation heat exchanger, the decompression structure, and the outdoor heat exchanger; wherein the air conditioner is configured to be operated in a heating operation mode for performing an indoor heating operation, the refrigerant circuit further comprising: a principal channel in which the decompression structure, the outdoor heat exchanger, and the compressor are provided in this order from an upstream side towards a downstream side during the heating operation, the principal channel extending between a branching section and a merging section; a first channel having the indoor heat exchanger, which is configured to connect the branching section on the downstream side of the compressor with the merging section provided on the upstream side of the decompression structure, during the heating operation; and a second channel having the radiation heat exchanger, which connects the branching section and the merging section in parallel with the first channel during the heating operation, a principal channel in which the decompression structure, the outdoor heat exchanger, and the compressor are provided in this order from an upstream side towards a downstream side during the heating operation, the principal channel extending between a branching section and a merging section; a first channel having the indoor heat exchanger, which is configured to connect the branching section on the downstream side of the compressor with the merging section provided on the upstream side of the decompression structure, during the heating operation; and a second channel having the radiation heat exchanger, which connects the branching section and the merging section in parallel with the first channel during the heating operation, a valve structure configured to adjust the amount of a refrigerant to be supplied to the radiation heat exchanger is provided to the refrigerant circuit; and wherein the air conditioner comprises a valve controller, the valve controller being configured to increase an opening degree of the valve structure after a frequency of the compressor is decreased, when a warm-air heating is performed by causing the refrigerant to flow in the indoor heat exchanger and a radiation heating is performed by causing the refrigerant to flow in the radiation heat exchanger so that a flow rate of the refrigerant flowing in the radiation heat exchanger and in the indoor heat exchanger is adjusted.