A heat pump system (1; 2; 3) comprising: a compression mechanism that includes a lower stage-side compressor and a higher stage-side compressor configured to compress and discharge a refrigerant; an oil separator that is provided on a discharge side of the higher stage-side compressor; a first heat exchanger configured to exchange heat between the refrigerant compressed by the compression mechanism and an object to be subjected to heat exchange; an expansion valve configured to decompress and expand the refrigerant flowing out of the first heat exchanger; a second heat exchanger configured to exchange heat between the refrigerant decompressed and expanded by the expansion valve and an object to be subjected to heat exchange; and a main return pipe that connects the oil separator to a suction side of the higher stage-side compressor; characterized by comprising an oil equalizing path that connects the lower stage-side compressor to the higher stage-side compressor configured to allow refrigerant oil to flow between the lower stage-side compressor and the higher stage-side compressor; an oil return path that connects the main return pipe to the oil equalizing path; an oil-return on-off valve that is provided on the oil return path; and a refrigerant path switching mechanism configured to selectively switch a refrigerant path between a two-stage compression path along which the refrigerant flows through the lower stage-side compressor and the higher stage-side compressor in this order and a one-stage compression path along which the refrigerant flows through only one of the lower stage-side compressor and the higher stage-side compressor, a control device configured to instruct the refrigerant path switching mechanism and the oil-return on-off valve such that when the control device instructs refrigerant path switching mechanism to select the one-stage compression path, the control device instructs the oil-return on-off valve to be opened and the refrigerant oil from the oil separator is returned to the oil equalizing path through the oil return path without passing through the higher stage-side compressor. a compression mechanism that includes a lower stage-side compressor and a higher stage-side compressor configured to compress and discharge a refrigerant; an oil separator that is provided on a discharge side of the higher stage-side compressor; a first heat exchanger configured to exchange heat between the refrigerant compressed by the compression mechanism and an object to be subjected to heat exchange; an expansion valve configured to decompress and expand the refrigerant flowing out of the first heat exchanger; a second heat exchanger configured to exchange heat between the refrigerant decompressed and expanded by the expansion valve and an object to be subjected to heat exchange; and a main return pipe that connects the oil separator to a suction side of the higher stage-side compressor; characterized by comprising an oil equalizing path that connects the lower stage-side compressor to the higher stage-side compressor configured to allow refrigerant oil to flow between the lower stage-side compressor and the higher stage-side compressor; an oil return path that connects the main return pipe to the oil equalizing path; an oil-return on-off valve that is provided on the oil return path; and a refrigerant path switching mechanism configured to selectively switch a refrigerant path between a two-stage compression path along which the refrigerant flows through the lower stage-side compressor and the higher stage-side compressor in this order and a one-stage compression path along which the refrigerant flows through only one of the lower stage-side compressor and the higher stage-side compressor, a control device configured to instruct the refrigerant path switching mechanism and the oil-return on-off valve such that when the control device instructs refrigerant path switching mechanism to select the one-stage compression path, the control device instructs the oil-return on-off valve to be opened and the refrigerant oil from the oil separator is returned to the oil equalizing path through the oil return path without passing through the higher stage-side compressor.