Refrigerant cycle system

A refrigerant cycle system comprising:

• a first refrigerant circuit that is a vapor compression refrigeration cycle;
• a second refrigerant circuit that is a vapor compression refrigeration cycle;
• a first cascade heat exchanger that exchanges heat between a first refrigerant that flows in the first refrigerant circuit and a second refrigerant that flows in the second refrigerant circuit,
• a first flow-rate regulating valve that regulates an amount of the first refrigerant that flows in the first cascade heat exchanger in the first refrigerant circuit, and
• a controller that regulates an opening degree of the first flow-rate regulating valve,
• wherein at least either one of the first refrigerant circuit and the second refrigerant circuit includes a switching valve that switches a flow path of a refrigerant of a circuit,
• the first cascade heat exchanger includes a first main heat exchanging unit and a first sub heat exchanging unit through which the first refrigerant that has passed through the first main heat exchanging unit passes,
• when the first cascade heat exchanger of the first refrigerant circuit serves as an evaporator, the controller regulates the opening degree of the first flow-rate regulating valve to cause the first refrigerant that exits the first sub heat exchanging unit to be in a superheating state, and
• the first main heat exchanging unit is a plate heat exchanger or a heat exchanger that includes a plurality of stacked flat pipes, wherein
• the first refrigerant circuit further includes a compressor and a heat-source-side heat exchanger,
• the second refrigerant circuit further includes a compressor, a usage-side heat exchanger and a second flow-rate regulating valve,
• when the first cascade heat exchanger of the first refrigerant circuit serves as an evaporator, in the first refrigerant circuit, the first refrigerant flows through the compressor, the heat-source-side heat exchanger, the first flow-rate regulating valve, the first main heat exchanging unit, and the first sub heat exchanging unit in this order, and
• when the first cascade heat exchanger of the first refrigerant circuit serves as an evaporator, in the second refrigerant circuit, the second refrigerant flows through the compressor, the first sub heat exchanging unit, the first main heat exchanging unit, a second flow-rate regulating valve and the usage-side heat exchanger in this order.