A refrigeration cycle apparatus (1, 51, 52), comprising: a refrigerant circuit connecting a first compressor, a first heat exchanger that serves as a radiator or a condenser, an expander, and a second heat exchanger that serves as an evaporator in series with a piping; a second compressor that is driven by power recovered by the expander, wherein the second compressor is being disposed i) between the first compressor and the first heat exchanger in the refrigerant circuit; or ii) between the first compressor and the second heat exchanger in the refrigerant circuit; characterised in that the second compressor is a positive displacement compressor, the refrigeration cycle apparatus further comprising a pressure regulating device that maintains a pressure on a discharge side of the second compressor to be lower than a pressure on a suction side of the second compressor at least until the second compressor is started up, wherein the pressure regulating device includes: a bypass (8, 11) having one end connected between the first compressor and the second compressor and the other end connected between the first heat exchanger and the expander; and an on-off valve (9, 12) provided in the bypass (8, 11), and wherein the on-off valve (9, 12) is maintained in an opened state at least until the second compressor is started up. a refrigerant circuit connecting a first compressor, a first heat exchanger that serves as a radiator or a condenser, an expander, and a second heat exchanger that serves as an evaporator in series with a piping; a second compressor that is driven by power recovered by the expander, wherein the second compressor is being disposed i) between the first compressor and the first heat exchanger in the refrigerant circuit; or ii) between the first compressor and the second heat exchanger in the refrigerant circuit; characterised in that the second compressor is a positive displacement compressor, i) between the first compressor and the first heat exchanger in the refrigerant circuit; or ii) between the first compressor and the second heat exchanger in the refrigerant circuit; characterised in that the second compressor is a positive displacement compressor, the refrigeration cycle apparatus further comprising a pressure regulating device that maintains a pressure on a discharge side of the second compressor to be lower than a pressure on a suction side of the second compressor at least until the second compressor is started up, wherein the pressure regulating device includes: a bypass (8, 11) having one end connected between the first compressor and the second compressor and the other end connected between the first heat exchanger and the expander; and an on-off valve (9, 12) provided in the bypass (8, 11), and wherein the on-off valve (9, 12) is maintained in an opened state at least until the second compressor is started up. a bypass (8, 11) having one end connected between the first compressor and the second compressor and the other end connected between the first heat exchanger and the expander; and an on-off valve (9, 12) provided in the bypass (8, 11), and wherein the on-off valve (9, 12) is maintained in an opened state at least until the second compressor is started up.