To provide a pressure control valve which conventionally controls pressure of CO2 as a refrigerant in a CO2 cycle including an inter-cooler and which is, as an improvement according to the present invention, capable of operation of the CO2 cycle at a high COP, compared with the prior art where there is not realized in a CO2 cycle one operating at high COP by controlling control temperature-pressure characteristics.

There are provided a temperature-pressure conversion mechanism, a pressure- force conversion mechanism, and a force-valve opening conversion mechanism. Each mechanism is actuated with gas cooler outlet temperature and inter-cooler outlet pressure. When an operation is carried out under the condition that CO2 as a refrigerant is in a supercritical pressure range, an outlet side valve is throttled to control the pressure to the outlet pressure that becomes a maximum COP corresponding to the outlet temperature. Upon operation in a range lower than the supercritical pressure, the valve is fully opened to lower the outlet temperature and hence reduce the amount of work required for a compressor for assurance of the operation of the cycle at the high COP. Hereby, when the refrigerant pressure is in the supercritical pressure zone there is ensured operation control where COP is made maximum with refrigerant pressure of the outlet pressure corresponding to the outlet side temperature, in the region less than the supercritical pressure, the pressure control valve is fully opened to ensure the operation control where condensation pressure is made higher than COP on thermal equilibrium of the present cycle.