A heat pump comprising a compressor, a switching valve, an indoor heat exchanger, a first expansion valve, and an outdoor heat exchanger, in which a refrigerant is flowable sequentially through the compressor, the switching valve, the outdoor heat exchanger, the first expansion valve, the indoor heat exchanger, the switching valve, and the compressor in cooling, and the refrigerant is flowable sequentially through the compressor, the switching valve, the indoor heat exchanger, the first expansion valve, the outdoor heat exchanger, the switching valve, and the compressor in heating, wherein the outdoor heat exchanger includes: a primary heat exchanging unit that is connected with the first expansion valve through a refrigerant pipe and for the outdoor air to exchange heat; a secondary heat exchanging unit that is connected with the switching valve through a refrigerant pipe and disposed behind the primary heat exchanging unit in the flow direction of outdoor air, and through which the outdoor air, which has exchanged heat through the primary heat exchanging unit, exchanges heat; a heat exchanging unit connection pipe that connects the primary heat exchanging unit and the secondary heat exchanging unit with each other; and a second expansion valve that is disposed in the heat exchanging unit connection pipe, wherein in a heating operation at an outdoor temperature over a predetermined temperature, the first expansion valve is configured to be controlled to open at a degree for expanding a refrigerant and the second expansion valve configured to be fully opened, and in a heating mode at an outdoor temperature less than a predetermined temperature, the first expansion valve configured to be fully opened and the second expansion valve is configured to be controlled to open at a degree for expanding a refrigerant, characterized in that the primary heat exchanging unit includes a primary microchannel heat exchanger having a pair of headers (21, 22) connected by a microchannel, and the secondary heat exchanging unit includes: a first secondary microchannel heat exchanger having a pair of headers (31, 32) connected by a microchannel and disposed behind the primary microchannel heat exchanger in the flow direction of outdoor air, and a second secondary microchannel heat exchanger disposed behind the first secondary microchannel heat exchanger in the flow direction of outdoor air, wherein the first secondary microchannel heat exchanger is connected with the primary microchannel heat exchanger through the heat exchanging unit connection pipe, in which the second expansion valve is disposed. a primary heat exchanging unit that is connected with the first expansion valve through a refrigerant pipe and for the outdoor air to exchange heat; a secondary heat exchanging unit that is connected with the switching valve through a refrigerant pipe and disposed behind the primary heat exchanging unit in the flow direction of outdoor air, and through which the outdoor air, which has exchanged heat through the primary heat exchanging unit, exchanges heat; a heat exchanging unit connection pipe that connects the primary heat exchanging unit and the secondary heat exchanging unit with each other; and a second expansion valve that is disposed in the heat exchanging unit connection pipe, wherein in a heating operation at an outdoor temperature over a predetermined temperature, the first expansion valve is configured to be controlled to open at a degree for expanding a refrigerant and the second expansion valve configured to be fully opened, and in a heating mode at an outdoor temperature less than a predetermined temperature, the first expansion valve configured to be fully opened and the second expansion valve is configured to be controlled to open at a degree for expanding a refrigerant, characterized in that the primary heat exchanging unit includes a primary microchannel heat exchanger having a pair of headers (21, 22) connected by a microchannel, and the secondary heat exchanging unit includes: a first secondary microchannel heat exchanger having a pair of headers (31, 32) connected by a microchannel and disposed behind the primary microchannel heat exchanger in the flow direction of outdoor air, and a second secondary microchannel heat exchanger disposed behind the first secondary microchannel heat exchanger in the flow direction of outdoor air, wherein the first secondary microchannel heat exchanger is connected with the primary microchannel heat exchanger through the heat exchanging unit connection pipe, in which the second expansion valve is disposed. a first secondary microchannel heat exchanger having a pair of headers (31, 32) connected by a microchannel and disposed behind the primary microchannel heat exchanger in the flow direction of outdoor air, and a second secondary microchannel heat exchanger disposed behind the first secondary microchannel heat exchanger in the flow direction of outdoor air, wherein the first secondary microchannel heat exchanger is connected with the primary microchannel heat exchanger through the heat exchanging unit connection pipe, in which the second expansion valve is disposed.