An air conditioner includes a compressor; a flow switching part disposed at an outlet side of the compressor to switch a flow direction of a refrigerant according to whether a cooling operation or heating operation is performed; an outdoor heat exchanger connected to the flow switching part and having a plurality of the refrigerant pipes to guide the refrigerant which heat-exchanges with outdoor air; and a main expansion valve connected to one side of the outdoor heat exchanger. A first inlet/outlet pipe connects the flow switching part to the outdoor heat exchanger; and a second inlet/outlet pipe connects the outdoor heat exchanger to the main expansion valve, where the outdoor heat exchanger includes a header to form a flowing space of the refrigerant and having an upper header and a lower header; a check valve disposed between the upper header and the lower header to guide the refrigerant to flow one way; and a bypass pipe connecting the lower header to the second inlet/outlet pipe and guiding a discharge of a liquid refrigerant located in the lower header, and a bypass pipe valve controlling an amount of the liquid refrigerant flowing through the bypass pipe.