REFRIGERATION CYCLE SYSTEM

To suppress an increase in pressure of a refrigerant on a discharge side of a compressor in a secondary-side refrigerant circuit. A refrigeration cycle system (1) includes a primary-side refrigerant circuit (5a) and a secondary-side refrigerant circuit (10). The secondary-side refrigerant circuit (10) includes a secondary-side compressor (21), a cascade heat exchanger (35), a secondary-side expansion valve (36), and a utilization-side heat exchanger (52a-c), which are connected to each other, and a carbon dioxide refrigerant circulates through the secondary-side refrigerant circuit. The secondary-side refrigerant circuit (10) includes a third heat source pipe (25) connecting the secondary-side compressor (21) to the cascade heat exchanger (35), a fourth heat source pipe (26) connecting the cascade heat exchanger (35) to the secondary-side expansion valve (36), a suction flow path (23), and a bypass flow path (47) connecting at least one of the third heat source pipe (25) and the fourth heat source pipe (26) to the suction flow path (23). The primary-side refrigerant circuit (5a) includes the cascade heat exchanger (35), and a heat medium different from the carbon dioxide refrigerant circulates through the primary-side refrigerant circuit. In the case of using the cascade heat exchanger (35) as a radiator of the secondary-side refrigerant circuit (10) and a heat sink of the primary-side refrigerant circuit (5a), the control unit (80) starts the secondary-side compressor (21) of the secondary-side refrigerant circuit (10) after a flow of the heat medium generates in the cascade heat exchanger (35) in the primary-side refrigerant circuit (5a).