A heat pump comprising: a low-stage compressor which is configured to compress a refrigerant; a high-stage compressor which is configured to compress the refrigerant discharged from the low-stage compressor; a condenser which is configured to condense the refrigerant discharged from the high-stage compressor; an expansion unit which is configured to depressurize the refrigerant supplied from the condenser; an evaporator which is connected to the expansion unit and configured to evaporate the refrigerant supplied from the expansion unit; a four-way valve which is configured to select a first channel through which the refrigerant discharged from the high-stage compressor is introduced into the condenser and the refrigerant supplied from the evaporator is introduced into the low-stage compressor and a second channel through which the refrigerant discharged from the high-stage compressor is introduced into the evaporator and the refrigerant supplied from the condenser is introduced into the low-stage compressor; a bypass flow path through which the refrigerant supplied from the evaporator is introduced into the low-stage compressor without passing through the four-way valve or through which the refrigerant supplied from the evaporator is introduced into the high-stage compressor without passing through the four-way valve and the low-stage compressor; and: an on-off valve which is provided in the bypass flow path and opens or closes the bypass flow path; and a controller which is configured to operate the on-off valve and the four-way valve, characterised in that the evaporator includes a first heat exchanger which is configured to perform heat exchange between the refrigerant and a first heat medium, and a second heat exchanger which is provided in parallel with the first heat exchanger and is configured to perform heat exchange between the refrigerant and a second heat medium, wherein the bypass flow path includes a first low-side bypass section which connects the first heat exchanger and the low-stage compressor to each other, a first high-side bypass section which connects the first heat exchanger and the high-stage compressor to each other without passing through the low-stage compressor, a second low-side bypass section which connects the second heat exchanger and the low-stage compressor to each other, and a second high-side bypass section which connects the second heat exchanger and the high-stage compressor to each other without passing through the low-stage compressor, wherein the on-off valve includes a first valve which is provided in the first low-side bypass section, a second valve which is provided in the first high-side bypass section, a third valve which is provided in the second low-side bypass section, and a fourth valve which is provided in the second high-side bypass section. a low-stage compressor which is configured to compress a refrigerant; a high-stage compressor which is configured to compress the refrigerant discharged from the low-stage compressor; a condenser which is configured to condense the refrigerant discharged from the high-stage compressor; an expansion unit which is configured to depressurize the refrigerant supplied from the condenser; an evaporator which is connected to the expansion unit and configured to evaporate the refrigerant supplied from the expansion unit; a four-way valve which is configured to select a first channel through which the refrigerant discharged from the high-stage compressor is introduced into the condenser and the refrigerant supplied from the evaporator is introduced into the low-stage compressor and a second channel through which the refrigerant discharged from the high-stage compressor is introduced into the evaporator and the refrigerant supplied from the condenser is introduced into the low-stage compressor; a bypass flow path through which the refrigerant supplied from the evaporator is introduced into the low-stage compressor without passing through the four-way valve or through which the refrigerant supplied from the evaporator is introduced into the high-stage compressor without passing through the four-way valve and the low-stage compressor; and: an on-off valve which is provided in the bypass flow path and opens or closes the bypass flow path; and a controller which is configured to operate the on-off valve and the four-way valve, characterised in that the evaporator includes a first heat exchanger which is configured to perform heat exchange between the refrigerant and a first heat medium, and a second heat exchanger which is provided in parallel with the first heat exchanger and is configured to perform heat exchange between the refrigerant and a second heat medium, wherein the bypass flow path includes a first low-side bypass section which connects the first heat exchanger and the low-stage compressor to each other, a first high-side bypass section which connects the first heat exchanger and the high-stage compressor to each other without passing through the low-stage compressor, a second low-side bypass section which connects the second heat exchanger and the low-stage compressor to each other, and a second high-side bypass section which connects the second heat exchanger and the high-stage compressor to each other without passing through the low-stage compressor, wherein the on-off valve includes a first valve which is provided in the first low-side bypass section, a second valve which is provided in the first high-side bypass section, a third valve which is provided in the second low-side bypass section, and a fourth valve which is provided in the second high-side bypass section.