An air conditioner may include an outdoor unit having a compressor; at least one indoor unit having an indoor heat exchanger, an indoor expansion valve, and a leak sensor that senses leakage of the refrigerant; a gaseous line; a liquid line; a first shut-off valve disposed adjacent to the at least one indoor unit that opens and closes the gaseous line; a second shut-off valve disposed adjacent to the at least one indoor unit that opens and closes the liquid line; a subcooling pipe branched from the liquid line and connected to an inlet of the compressor; a subcooling expansion valve that expands refrigerant, flowing through the subcooling pipe; and a controller configured to control operation of the compressor and to control opening and closing of the first shut-off valve, the second shut-off valve, the indoor expansion valve, and the subcooling expansion valve.