An object of the present invention is to improve a heat exchanging capability in an evaporator in a refrigerant cycle system in which a high pressure side is operated at a supercritical pressure. The refrigerant cycle system is a refrigerant cycle system in which a compressor, a gas cooler, an expansion valve and an evaporator are sequentially connected in a cyclic form and a high pressure side is operated at a supercritical pressure, wherein the degree of opening of the expansion valve is adjusted based on the temperature and pressure of a refrigerant at an outlet of the evaporator so as to control the degree of superheat at the outlet of the evaporator. An abstract of the present invention is to make large the degree of superheat at the outlet of the evaporator by means of the expansion valve.