An air conditioner comprising: a main body comprising a suction hole for suctioning air and a discharge hole for discharging the air; a compressor disposed in the main body to compress a refrigerant; a heat exchanger receiving the refrigerant compressed by the compressor, the heat exchanger comprising a plurality of heat-exchange parts (21, 22, 23), wherein the plurality of heat-exchange parts comprises: a first heat-exchange part; a second heat-exchange part disposed at a downstream side of the first heat-exchange part; and a third heat-exchange part disposed at a downstream side of the second heat-exchange part; an expander disposed in the main body to expand the refrigerant; and a valve (50; 80) to control a flow of the refrigerant into the heat exchanger, characterized in that: the refrigerant compressed in the compressor is supplied into the second heat-exchange part, and the valve controls the flow of the refrigerant so that the refrigerant discharged from the second heat-exchange part flows into at least one of the first heat-exchange part and the third heat-exchange part, an operation mode of the air conditioner comprises a cooling mode and a dehumidifying mode, the cooling mode including a first mode and a second mode, wherein the plurality of heat-exchange parts (21, 22, 23) are disposed in series with respect to a flow direction of the air to allow the air to successively pass therethrough, and the valve (50; 80) controls the flow of the refrigerant such that: in the cooling mode, a portion of the plurality of heat-exchange parts (21, 22, 23) acts as a condenser and another portion (21, 23) of the plurality of heat-exchange parts acts as an evaporator so that the air passes through the heat-exchange part that acts as the evaporator after passing through the heat-exchange part that acts as the condenser, and in the dehumidifying mode, a portion of the plurality of heat-exchange parts (21, 22, 23) acts as an evaporator and another portion of the plurality of heat-exchange parts acts as a condenser so that the air passes through the heat-exchange part that acts as the condenser after passing through the heat-exchange part that acts the evaporator, wherein, in the first mode, the valve controls the flow of the refrigerant so that the refrigerant discharged from the expander flows into each of the first and third heat-exchange parts (21, 23), and in the second mode, the valve controls the flow of the refrigerant so that the refrigerant discharged from the expander flows into the third heat-exchange part. a main body comprising a suction hole for suctioning air and a discharge hole for discharging the air; a compressor disposed in the main body to compress a refrigerant; a heat exchanger receiving the refrigerant compressed by the compressor, the heat exchanger comprising a plurality of heat-exchange parts (21, 22, 23), wherein the plurality of heat-exchange parts comprises: a first heat-exchange part; a second heat-exchange part disposed at a downstream side of the first heat-exchange part; and a third heat-exchange part disposed at a downstream side of the second heat-exchange part; an expander disposed in the main body to expand the refrigerant; and a valve (50; 80) to control a flow of the refrigerant into the heat exchanger, the refrigerant compressed in the compressor is supplied into the second heat-exchange part, and the valve controls the flow of the refrigerant so that the refrigerant discharged from the second heat-exchange part flows into at least one of the first heat-exchange part and the third heat-exchange part, an operation mode of the air conditioner comprises a cooling mode and a dehumidifying mode, the cooling mode including a first mode and a second mode, wherein the plurality of heat-exchange parts (21, 22, 23) are disposed in series with respect to a flow direction of the air to allow the air to successively pass therethrough, and the valve (50; 80) controls the flow of the refrigerant such that: in the cooling mode, a portion of the plurality of heat-exchange parts (21, 22, 23) acts as a condenser and another portion (21, 23) of the plurality of heat-exchange parts acts as an evaporator so that the air passes through the heat-exchange part that acts as the evaporator after passing through the heat-exchange part that acts as the condenser, and in the dehumidifying mode, a portion of the plurality of heat-exchange parts (21, 22, 23) acts as an evaporator and another portion of the plurality of heat-exchange parts acts as a condenser so that the air passes through the heat-exchange part that acts as the condenser after passing through the heat-exchange part that acts the evaporator, wherein, in the first mode, the valve controls the flow of the refrigerant so that the refrigerant discharged from the expander flows into each of the first and third heat-exchange parts (21, 23), and in the second mode, the valve controls the flow of the refrigerant so that the refrigerant discharged from the expander flows into the third heat-exchange part.