Ejector cycle system with critical refrigerant pressure

In an ejector cycle system using carbon dioxide as refrigerant, an ejector (400) decompresses and expands refrigerant from a radiator (200) to suck gas refrigerant evaporated in an evaporator (300), and converts an expansion energy to a pressure energy to increase a refrigerant pressure to be sucked into a compressor (100). A control valve (731) is disposed in a refrigerant passage between the radiator (200) and the ejector (400) to control an opening degree of the refrigerant passage based on a heating degree of refrigerant at an outlet side of the evaporator (300). Because refrigerant is decompressed and expanded in a super-critical area, a pressure difference during the decompression operation becomes larger, and a specific enthalpy difference becomes larger. Accordingly, energy converting efficiency in the ejector becomes higher, and efficiency of the ejector cycle system is improved.