Supercritical refrigeration cycle system has

A CO 2 cycle system having a variable capacity compressor (1) is controlled even when refrigerant discharge capacity (Qd) of the compressor (1) changes. When the refrigerant discharge capacity (Qd) decreases, the opening of an expansion valve (3) is fixed for a predetermined time To. When the refrigerant discharge capacity (Qd) did not change, the opening the solenoid valve (3) is controlled so that the temperature and pressure of the refrigerant at the outlet of a heat exchanger (2) of the system vary according to an optimum control curve. As the refrigerant discharge capacity (Qd) is controlled based on the suction pressure (Ps) prevailing on the side of the evaporator (4) without being influenced by controlling the side of the valve (3), the system CO2 cycle can be controlled appropriately.