A refrigeration system including a refrigerant circuit that has, in order to perform a vapor compression supercritical refrigeration cycle, a compression mechanism, a heat source side heat exchanger, an expansion mechanism, and a utilization side heat exchanger, wherein the expansion mechanism includes, for two-stage expansion of refrigerant in the refrigerant circuit, a high pressure side throttle mechanism (41, 42) variable in the amount of throttling and a low pressure side throttle mechanism (42, 41) variable in the amount of throttling; characterized in that said refrigeration system comprises a high pressure control means which is configured to for each temperature of a medium exchanging heat with refrigerant in either the heat source side heat exchanger or the utilization side heat exchanger, which becomes a heat dissipation side heat exchanger functioning as a heat dissipation unit, at the inlet of the heat dissipation side heat exchanger, determine a relationship between the temperature of refrigerant at the outlet of the heat dissipation side heat exchanger and the pressure of high pressure refrigerant in the refrigerant circuit, for each temperature of the medium at the inlet of the heat dissipation side heat exchanger, determine a relationship between a coefficient of performance and the temperature of refrigerant at the outlet of the heat dissipation side heat exchanger and a relationship between the coefficient of performance and the pressure of the high pressure refrigerant in the refrigerant circuit, and derive a target value, providing a maximum coefficient of performance, for the pressure of high pressure refrigerant in the refrigerant circuit based on i) the temperature of refrigerant at the outlet of the heat dissipation side heat exchanger and ii) the temperature of the medium at the inlet of the heat dissipation side heat exchanger; and the high pressure control means is further configured to adjust the amount of throttling of the expansion mechanism so that the high pressure refrigerant pressure is controlled to the derived target value, and/or said refrigeration system comprises an outlet temperature control means which is configured to for each temperature of a medium exchanging heat with refrigerant in the utilization side heat exchanger functioning as a heat dissipation unit, at the inlet of the utilization side heat exchanger, determine a relationship between the temperature of refrigerant at the outlet of the utilization side heat exchanger and the pressure of high pressure refrigerant in the refrigerant circuit, for each temperature of the medium at the inlet of the utilization side heat exchanger, determine a relationship between a coefficient of performance and the temperature of refrigerant at the outlet of the utilization side heat exchanger and a relationship between the coefficient of performance and the pressure of the high pressure refrigerant in the refrigerant circuit, and derive, in a heating operation mode of the refrigerant circuit, a target value providing a maximum coefficient of performance for the temperature of refrigerant at the outlet of the utilization side heat exchanger based on i) the temperature of the medium at the inlet of the utilization side heat exchanger and ii) a preset pressure value for the pressure of high pressure refrigerant in the refrigerant circuit, and the outlet temperature control means is further configured to adjust the amount of throttling of the expansion mechanism so that the outlet refrigerant temperature is controlled to the target value. i) the temperature of refrigerant at the outlet of the heat dissipation side heat exchanger and ii) the temperature of the medium at the inlet of the heat dissipation side heat exchanger; and the high pressure control means is further configured to adjust the amount of throttling of the expansion mechanism so that the high pressure refrigerant pressure is controlled to the derived target value, and/or i) the temperature of the medium at the inlet of the utilization side heat exchanger and ii) a preset pressure value for the pressure of high pressure refrigerant in the refrigerant circuit, and the outlet temperature control means is further configured to adjust the amount of throttling of the expansion mechanism so that the outlet refrigerant temperature is controlled to the target value.