An ejector cycle system comprising: a compressor for sucking and compressing refrigerant; a radiator for cooling the refrigerant discharged from the compressor; an evaporator in which refrigerant is evaporated by absorbing heat; an ejector which decompresses and expands the refrigerant from the radiator to suck gas refrigerant evaporated in the evaporator, and converts an expansion energy to a pressure energy to increase the refrigerant pressure to be sucked into the compressor; and a gas-liquid separator for storing refrigerant and for separating the refrigerant from the ejector into gas refrigerant and liquid refrigerant, wherein the gas-liquid separator is disposed to separate gas refrigerant and liquid refrigerant from each other so that gas refrigerant in the gas-liquid separator is supplied to a suction side of the compressor and the liquid refrigerant in the gas-liquid separator is supplied to the evaporator, the ejector cycle system being characterized by further comprising: a control valve which is disposed in a refrigerant passage between the radiator and the ejector having a super-heat sensing means (731a, 731b) sensing the refrigerant temperature at the refrigerant outlet side of the evaporator and controls an opening degree of the refrigerant passage based on a predetermined super-heating degree of refrigerant at an outlet side of the evaporator so that the ejector efficiency becomes maximum at the predetermined super-heating degree, wherein the ejector cycle system being adapted to when in use to have the refrigerant pressure, before being decompressed in the ejector, equal to or higher than the critical pressure of the refrigerant, the refrigerant is carbon dioxide, ethylene or ethane, and the refrigerant pressure at the outlet side of the evaporator is lower than the critical pressure of the refrigerant. a compressor for sucking and compressing refrigerant; a radiator for cooling the refrigerant discharged from the compressor; an evaporator in which refrigerant is evaporated by absorbing heat; an ejector which decompresses and expands the refrigerant from the radiator to suck gas refrigerant evaporated in the evaporator, and converts an expansion energy to a pressure energy to increase the refrigerant pressure to be sucked into the compressor; and a gas-liquid separator for storing refrigerant and for separating the refrigerant from the ejector into gas refrigerant and liquid refrigerant, wherein the gas-liquid separator is disposed to separate gas refrigerant and liquid refrigerant from each other so that gas refrigerant in the gas-liquid separator is supplied to a suction side of the compressor and the liquid refrigerant in the gas-liquid separator is supplied to the evaporator, the ejector cycle system being characterized by further comprising: a control valve which is disposed in a refrigerant passage between the radiator and the ejector having a super-heat sensing means (731a, 731b) sensing the refrigerant temperature at the refrigerant outlet side of the evaporator and controls an opening degree of the refrigerant passage based on a predetermined super-heating degree of refrigerant at an outlet side of the evaporator so that the ejector efficiency becomes maximum at the predetermined super-heating degree, wherein the ejector cycle system being adapted to when in use to have the refrigerant pressure, before being decompressed in the ejector, equal to or higher than the critical pressure of the refrigerant, the refrigerant is carbon dioxide, ethylene or ethane, and the refrigerant pressure at the outlet side of the evaporator is lower than the critical pressure of the refrigerant. a control valve which is disposed in a refrigerant passage between the radiator and the ejector having a super-heat sensing means (731a, 731b) sensing the refrigerant temperature at the refrigerant outlet side of the evaporator and controls an opening degree of the refrigerant passage based on a predetermined super-heating degree of refrigerant at an outlet side of the evaporator so that the ejector efficiency becomes maximum at the predetermined super-heating degree, wherein the ejector cycle system being adapted to when in use to have the refrigerant pressure, before being decompressed in the ejector, equal to or higher than the critical pressure of the refrigerant, the refrigerant is carbon dioxide, ethylene or ethane, and the refrigerant pressure at the outlet side of the evaporator is lower than the critical pressure of the refrigerant.