A refrigeration cycle system comprising: a first cycle; and a second cycle, wherein the first cycle is connected with a first compressor, a cascade heat exchanger, a first expansion unit (36, 11), and a first heat exchanger (52a, 52b, 52c), has a carbon dioxide refrigerant circulating through the first cycle, and includes a first flow path that connects the first compressor to the cascade heat exchanger, a second flow path that connects the cascade heat exchanger to the first expansion unit (36, 11), a third flow path that connects the first heat exchanger (52a, 52b, 52c) to the first compressor, and a bypass flow path (47, 47a, 41, 41a) that connects at least one of the first flow path and the second flow path to the third flow path, wherein the second cycle includes the cascade heat exchanger, and has a heat medium different from the carbon dioxide refrigerant circulating through the second cycle, wherein the bypass flow path (47, 47a, 41, 41a) includes an on-off valve that can be opened and closed, characterized in that the refrigeration cycle system comprises a heat source-side control unit, wherein the heat source-side control unit is configured to control the first compressor, such that in a case of using the cascade heat exchanger as a radiator of the first cycle and a heat sink of the second cycle, the first compressor of the first cycle is started after a flow of the heat medium generates in the cascade heat exchanger in the second cycle, and wherein the heat source-side control unit is configured to control the on-off valve, such that in the case of using the cascade heat exchanger as the radiator of the first cycle and the heat sink of the second cycle, the on-off valve is in an open state from after the heat medium starts to flow in the cascade heat exchanger in the second cycle until the first compressor is started, and the on-off valve is switched to a close state when or after the first compressor is started. a first cycle; and a second cycle, wherein the first cycle is connected with a first compressor, a cascade heat exchanger, a first expansion unit (36, 11), and a first heat exchanger (52a, 52b, 52c), has a carbon dioxide refrigerant circulating through the first cycle, and includes a first flow path that connects the first compressor to the cascade heat exchanger, a second flow path that connects the cascade heat exchanger to the first expansion unit (36, 11), a third flow path that connects the first heat exchanger (52a, 52b, 52c) to the first compressor, and a bypass flow path (47, 47a, 41, 41a) that connects at least one of the first flow path and the second flow path to the third flow path, wherein the second cycle includes the cascade heat exchanger, and has a heat medium different from the carbon dioxide refrigerant circulating through the second cycle, wherein the bypass flow path (47, 47a, 41, 41a) includes an on-off valve that can be opened and closed, characterized in that the refrigeration cycle system comprises a heat source-side control unit, wherein the heat source-side control unit is configured to control the first compressor, such that in a case of using the cascade heat exchanger as a radiator of the first cycle and a heat sink of the second cycle, the first compressor of the first cycle is started after a flow of the heat medium generates in the cascade heat exchanger in the second cycle, and wherein the heat source-side control unit is configured to control the on-off valve, such that in the case of using the cascade heat exchanger as the radiator of the first cycle and the heat sink of the second cycle, the on-off valve is in an open state from after the heat medium starts to flow in the cascade heat exchanger in the second cycle until the first compressor is started, and the on-off valve is switched to a close state when or after the first compressor is started.