An air conditioner may include a case through which outdoor air flows into an indoor space is formed; a first heat exchanger that exchanges heat between the air and refrigerant; a second heat exchanger disposed downstream of the first heat exchanger, through which the refrigerant selectively flows, and that exchanges heat between the air and the refrigerant; a liquid pipe connected to each of the first and second heat exchangers; a high-pressure gas pipe connected to each of the first and second heat exchangers; a low-pressure gas pipe through which the gaseous refrigerant discharged from the first and second heat exchangers flows; a high-pressure gas pipe valve; a low-pressure gas pipe valve; an expansion valve installed in the liquid pipe; and a controller configured to control an opening degree of the low-pressure gas pipe valve, based on a temperature of the refrigerant flowing through the first heat exchanger.