An air conditioner comprising: a compressor (121, 1121) configured to compress a refrigerant; an outdoor heat exchanger configured to perform heat exchange between outdoor air and the refrigerant; an indoor heat exchanger configured to perform heat exchange between indoor air and the refrigerant; a four-way valve (123, 1123) configured to guide the refrigerant compressed in the compressor (121, 1121) to either the outdoor heat exchanger or the indoor heat exchanger according to a cooling mode or a heating mode; an indoor temperature measurer configured to measure an indoor temperature; and a controller (131, 1131) configured to: change the heating mode to the cooling mode when the measured indoor temperature is maintained at a first reference indoor temperature or greater for a determined first duration, and change the cooling mode to the heating mode when the measured indoor temperature is maintained at a second reference indoor temperature or less for a determined second duration, and characterized in that the air conditioner further comprises an outdoor temperature measurer (134, 1134) configured to measure an outdoor temperature; and in that the controller (131, 1131) is further configured to: determine the first duration based on the measured outdoor temperature when the air conditioner is operating in the heating mode or determine the second duration based on the measured outdoor temperature when the air conditioner is operating in the cooling mode. a compressor (121, 1121) configured to compress a refrigerant; an outdoor heat exchanger configured to perform heat exchange between outdoor air and the refrigerant; an indoor heat exchanger configured to perform heat exchange between indoor air and the refrigerant; a four-way valve (123, 1123) configured to guide the refrigerant compressed in the compressor (121, 1121) to either the outdoor heat exchanger or the indoor heat exchanger according to a cooling mode or a heating mode; an indoor temperature measurer configured to measure an indoor temperature; and a controller (131, 1131) configured to: change the heating mode to the cooling mode when the measured indoor temperature is maintained at a first reference indoor temperature or greater for a determined first duration, and change the cooling mode to the heating mode when the measured indoor temperature is maintained at a second reference indoor temperature or less for a determined second duration, and characterized in that the air conditioner further comprises an outdoor temperature measurer (134, 1134) configured to measure an outdoor temperature; and in that the controller (131, 1131) is further configured to: determine the first duration based on the measured outdoor temperature when the air conditioner is operating in the heating mode or determine the second duration based on the measured outdoor temperature when the air conditioner is operating in the cooling mode. change the heating mode to the cooling mode when the measured indoor temperature is maintained at a first reference indoor temperature or greater for a determined first duration, and change the cooling mode to the heating mode when the measured indoor temperature is maintained at a second reference indoor temperature or less for a determined second duration, and characterized in that the air conditioner further comprises an outdoor temperature measurer (134, 1134) configured to measure an outdoor temperature; and in that the controller (131, 1131) is further configured to: determine the first duration based on the measured outdoor temperature when the air conditioner is operating in the heating mode or determine the second duration based on the measured outdoor temperature when the air conditioner is operating in the cooling mode.