An air conditioner comprising: an outdoor unit including a compressor, an outdoor heat exchanger, a first pressure sensor and a second pressure sensor, wherein the first pressure sensor is configured to detect a discharge pressure (Pd) on the discharge pressure side of the compressor and the second pressure sensor is configured to detect a suction pressure (Ps) on the suction side of the compressor; an indoor unit including an indoor heat exchanger and a decompression device; a timer; and a pipe (51, 52) connecting the outdoor unit and the indoor unit, wherein the outdoor unit includes a bypass path connecting a discharge side of the compressor and a suction side of the compressor, an on-off valve configured to open/close the bypass path, and a control device configured to control the compressor, the decompression device, and the on-off valve, and the control device being configured to open the on-off valve in a state in which the compressor is stopped to execute such bypass opening that refrigerant circulates, through the bypass path, from the discharge side of the compressor in a refrigerant storage state in which refrigerant is stored to the suction side of the compressor in a substantially vacuum state, and to start the timer, and to evaluate a volume of the pipe (51, 52) connecting the outdoor unit and the indoor unit based on the discharge pressure (Pd) on the discharge side of the compressor, a pressure change on the suction side of the compressor and a time required for the pressure change on the suction side of the compressor in the bypass opening. an outdoor unit including a compressor, an outdoor heat exchanger, a first pressure sensor and a second pressure sensor, wherein the first pressure sensor is configured to detect a discharge pressure (Pd) on the discharge pressure side of the compressor and the second pressure sensor is configured to detect a suction pressure (Ps) on the suction side of the compressor; an indoor unit including an indoor heat exchanger and a decompression device; a timer; and a pipe (51, 52) connecting the outdoor unit and the indoor unit, wherein the outdoor unit includes a bypass path connecting a discharge side of the compressor and a suction side of the compressor, an on-off valve configured to open/close the bypass path, and a control device configured to control the compressor, the decompression device, and the on-off valve, and a bypass path connecting a discharge side of the compressor and a suction side of the compressor, an on-off valve configured to open/close the bypass path, and a control device configured to control the compressor, the decompression device, and the on-off valve, and the control device being configured to open the on-off valve in a state in which the compressor is stopped to execute such bypass opening that refrigerant circulates, through the bypass path, from the discharge side of the compressor in a refrigerant storage state in which refrigerant is stored to the suction side of the compressor in a substantially vacuum state, and to start the timer, and to evaluate a volume of the pipe (51, 52) connecting the outdoor unit and the indoor unit based on the discharge pressure (Pd) on the discharge side of the compressor, a pressure change on the suction side of the compressor and a time required for the pressure change on the suction side of the compressor in the bypass opening.