An air conditioner comprising: a compressor arranged to compress refrigerant; an indoor heat exchanger arranged to heat exchange between the refrigerant passed through the compressor and indoor air; an expansion device arranged to decompress the refrigerant passed through the indoor heat exchanger; an outdoor heat exchanger arranged to heat exchange between the refrigerant supplied from the expansion device and outdoor air; a gas liquid separator provided at an inlet of the compressor to separate liquid refrigerant; a bypass path arranged to bypass the refrigerant from an outlet of the compressor toward at least one of an inlet of the outdoor heat exchanger and an inlet of the gas liquid separator; a first valve disposed on the bypass path to control a flow of the refrigerant; a plurality of sensors (110, 120, 130) arranged to sense temperature of the outdoor heat exchanger, indoor temperature and outdoor temperature, respectively; a heater arranged to generate heat variably according to the outdoor temperature and the outdoor heat exchanger temperature detected by the sensors, the heater comprising an induction heater located at the outside of the gas liquid separator; and a controller arranged to determine a quantity indicative of the amount of frost formed on the outdoor heat exchanger by comparing a reference temperature with a temperature difference between the outdoor temperature and the outdoor heat exchanger temperature, and controls output of the heater according to the determined frost quantity, wherein the output of the heater is increased as the temperature difference increases and the indoor temperature is greater than a first preset temperature. a compressor arranged to compress refrigerant; an indoor heat exchanger arranged to heat exchange between the refrigerant passed through the compressor and indoor air; an expansion device arranged to decompress the refrigerant passed through the indoor heat exchanger; an outdoor heat exchanger arranged to heat exchange between the refrigerant supplied from the expansion device and outdoor air; a gas liquid separator provided at an inlet of the compressor to separate liquid refrigerant; a bypass path arranged to bypass the refrigerant from an outlet of the compressor toward at least one of an inlet of the outdoor heat exchanger and an inlet of the gas liquid separator; a first valve disposed on the bypass path to control a flow of the refrigerant; a plurality of sensors (110, 120, 130) arranged to sense temperature of the outdoor heat exchanger, indoor temperature and outdoor temperature, respectively; a heater arranged to generate heat variably according to the outdoor temperature and the outdoor heat exchanger temperature detected by the sensors, the heater comprising an induction heater located at the outside of the gas liquid separator; and a controller arranged to determine a quantity indicative of the amount of frost formed on the outdoor heat exchanger by comparing a reference temperature with a temperature difference between the outdoor temperature and the outdoor heat exchanger temperature, and controls output of the heater according to the determined frost quantity, wherein the output of the heater is increased as the temperature difference increases and the indoor temperature is greater than a first preset temperature.