Refrigeration cycle system

A refrigeration cycle system comprising:

• a first circuit configured to circulate a first refrigerant and including a first compressor, a cascade heat exchanger, a heat source heat exchanger, and first switching valves configured to switch a flow path of the first refrigerant;
• a second circuit configured to circulate a second refrigerant and including a second compressor, the cascade heat exchanger, a utilization heat exchanger;
• and second switching valves configured to switch a flow path of the second refrigerant;
• wherein
• the second circuit includes a bypass circuit connecting a portion between the utilization heat exchanger and the cascade heat exchanger and a suction flow path of the second compressor, and a controlling valve provided on the bypass circuit;
• and
• an electronic controller configured to control the system to executes defrosting operation of circulating the first refrigerant in the order of the first compressor, the heat source heat exchanger, and the cascade heat exchanger, and circulating the second refrigerant in the order of the second compressor, the cascade heat exchanger, and the bypass circuit.