An air conditioner comprising: a compressor for compressing refrigerant; an indoor heat exchanger for cooling or heating a room, using the refrigerant; an outdoor heat exchanger for heat-exchanging the refrigerant with an outdoor air, the outdoor heat exchanger comprising a front heat exchange member and a rear heat exchange member; a defrosting module for adjusting an opening degree of a passage of the refrigerant introduced into the front heat exchange member and the rear heat exchange member; and a controller for controlling the defrosting module to operate the front heat exchange member as a condenser and to operate the rear heat exchange member as an evaporator when frost occurs only on the front heat exchange member in a heating mode, wherein the defrosting module comprises, a first expansion valve to adjust an opening of a passage of the refrigerant introduced into the front heat exchange member; and a defrosting unit to adjust an opening degree of a passage of the refrigerant between the front heat exchange member and the rear heat exchange member, wherein the defrosting unit includes a second expansion valve, and wherein when frost occurs on the front heat exchange member or the rear heat exchange member in a heating mode, the controller increases an opening of the first expansion valve to flow a high temperature refrigerant into the front heat exchange member and simultaneously decreases an opening degree of the defrosting unit to expand the refrigerant after passing the front heat exchange member into a low temperature refrigerant, only to operate the front heat exchange member as the condenser, the air conditioner further comprising: an outdoor fan to control the flow of outdoor air heat-exchanged in the outdoor heat exchanger, wherein when only the front heat exchange member is operated as a condenser to implement a defrosting mode, the controller controls the outdoor fan to prevent vapors or water from the front heat exchange member from flowing into the rear heat exchange member. a compressor for compressing refrigerant; an indoor heat exchanger for cooling or heating a room, using the refrigerant; an outdoor heat exchanger for heat-exchanging the refrigerant with an outdoor air, the outdoor heat exchanger comprising a front heat exchange member and a rear heat exchange member; a defrosting module for adjusting an opening degree of a passage of the refrigerant introduced into the front heat exchange member and the rear heat exchange member; and a controller for controlling the defrosting module to operate the front heat exchange member as a condenser and to operate the rear heat exchange member as an evaporator when frost occurs only on the front heat exchange member in a heating mode, wherein the defrosting module comprises, a first expansion valve to adjust an opening of a passage of the refrigerant introduced into the front heat exchange member; and a defrosting unit to adjust an opening degree of a passage of the refrigerant between the front heat exchange member and the rear heat exchange member, wherein the defrosting unit includes a second expansion valve, and wherein when frost occurs on the front heat exchange member or the rear heat exchange member in a heating mode, the controller increases an opening of the first expansion valve to flow a high temperature refrigerant into the front heat exchange member and simultaneously decreases an opening degree of the defrosting unit to expand the refrigerant after passing the front heat exchange member into a low temperature refrigerant, only to operate the front heat exchange member as the condenser, the air conditioner further comprising: an outdoor fan to control the flow of outdoor air heat-exchanged in the outdoor heat exchanger, wherein when only the front heat exchange member is operated as a condenser to implement a defrosting mode, the controller controls the outdoor fan to prevent vapors or water from the front heat exchange member from flowing into the rear heat exchange member. an outdoor fan to control the flow of outdoor air heat-exchanged in the outdoor heat exchanger, wherein when only the front heat exchange member is operated as a condenser to implement a defrosting mode, the controller controls the outdoor fan to prevent vapors or water from the front heat exchange member from flowing into the rear heat exchange member.