An air conditioner includes a compressor, first and second heat exchangers connected with high pressure piping, low pressure piping connecting the second heat exchanger to a compressor suction port, a pressure reducing mechanism arranged to reduce pressure in the high pressure piping, a bypass passageway, a vessel connected to the bypass passageway, and first and second opening/closing mechanisms. The first heat exchanger is connected to a compressor discharge port. The bypass passageway is arranged to divert refrigerant from the high pressure piping to the low pressure piping without passing through the second heat exchanger. The first opening/closing mechanism is arranged to open/close a first portion of the bypass passageway that connects the high pressure piping to the vessel. The second opening/closing mechanism is arranged to open/close a second portion of the bypass passageway that connects an upper part of the vessel to the low pressure piping.