Heat cycle system

A heat cycle system includes: a refrigerating cycle system 1000 that circulates a refrigerant; a medium circulation circuit 2000, 3000 that includes a circulation pump 5A, 5B for circulating a heat-transfer medium, and adjusts a temperature of a temperature control target with the heat-transfer medium; a heat exchanger 10A, 10B that executes heat exchange between the refrigerant in the refrigerating cycle system 1000 and the heat-transfer medium in the medium circulation circuit; and a volume altering unit 8 that alters a volume of the heat-transfer medium circulating in the medium circulation circuit.