A heat pump apparatus comprising: a refrigerant circuit in which a compressor, a first heat exchanger, an expansion mechanism, and a second heat exchanger are connected sequentially; a switching unit that switches between a heating operation and a defrost operation such that during the heating operation a refrigerant circulates through, in order, the compressor, the first heat exchanger, the expansion mechanism, and the second heat exchanger, and during the defrost operation the refrigerant circulates through, in order, the compressor, the second heat exchanger, the expansion mechanism, and the first heat exchanger; a superheat detection unit that detects a degree of superheat of the refrigerant at the first heat exchanger during the defrost operation; and a control unit that, when the heating operation is switched to the defrost operation by the switching unit, increases a circulation amount of the refrigerant circulating through the refrigerant circuit, and controls an operating frequency of the compressor such that the degree of superheat detected by the superheat detection unit is at a target value which is predetermined; and a bypass circuit that includes an on-off valve, and connects a midpoint between the first heat exchanger and the expansion mechanism with a midpoint between the expansion mechanism and the second heat exchanger in the refrigerant circuit such that the expansion mechanism is bypassed, wherein the expansion mechanism is configured by a first expansion mechanism, a liquid accumulator, and a second expansion mechanism connected sequentially in a direction from the first heat exchanger to the second heat exchanger, characterized in that the control unit, when the heating operation is switched to the defrost operation, opens the first expansion mechanism and closes the second expansion mechanism, and after a predetermined period of time has elapsed, closes the first expansion mechanism and opens the on-off valve included in the bypass circuit. a refrigerant circuit in which a compressor, a first heat exchanger, an expansion mechanism, and a second heat exchanger are connected sequentially; a switching unit that switches between a heating operation and a defrost operation such that during the heating operation a refrigerant circulates through, in order, the compressor, the first heat exchanger, the expansion mechanism, and the second heat exchanger, and during the defrost operation the refrigerant circulates through, in order, the compressor, the second heat exchanger, the expansion mechanism, and the first heat exchanger; a superheat detection unit that detects a degree of superheat of the refrigerant at the first heat exchanger during the defrost operation; and a control unit that, when the heating operation is switched to the defrost operation by the switching unit, increases a circulation amount of the refrigerant circulating through the refrigerant circuit, and controls an operating frequency of the compressor such that the degree of superheat detected by the superheat detection unit is at a target value which is predetermined; and a bypass circuit that includes an on-off valve, and connects a midpoint between the first heat exchanger and the expansion mechanism with a midpoint between the expansion mechanism and the second heat exchanger in the refrigerant circuit such that the expansion mechanism is bypassed, wherein the expansion mechanism is configured by a first expansion mechanism, a liquid accumulator, and a second expansion mechanism connected sequentially in a direction from the first heat exchanger to the second heat exchanger,