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). 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.