Abstract: In a heat pump device in which a compressor, a gas cooler, a refrigerant heat exchanger, a refrigerant expansion valve, and an evaporator are connected to configure a refrigerant circulation circuit, the heat pump device includes a buffer tank, one end being connected to the high-pressure side of the refrigerant expansion valve and arranged to store a refrigerant, and a first refrigerant pipe, one end being connected to the high-pressure side of the compressor and the other end connected to the low-pressure side of the evaporator and arranged to exchange heat with the buffer tank. The first refrigerant pipe includes a first control valve arranged between the high-pressure side of the compressor and the buffer tank to control opening and closing of the first refrigerant pipe, and a first flow rate regulator arranged between the buffer tank and the low-pressure side of the evaporator to control the refrigerant flow rate.