An engine exhaust gas purification system having an exhaust gas reactor mounted on the engine exhaust pipe, a secondary air supply system having a secondary air supply passage connected to the exhaust pipe upstream of the reactor, and an exhaust gas air-fuel ratio detector mounted on the exhaust pipe to detect air-fuel ratio of exhaust gases in the exhaust pipe. A secondary air supply control valve is provided in the secondary air supply passage and controlled by a valve actuator having a diaphragm to which positive pressure of the secondary air and the engine intake vacuum pressure can be applied. The application of the positive and vacuum pressures to the diaphragm is controlled by solenoid valves which are controlled in accordance with the air-fuel detector output voltage which represents the detected exhaust gas air-fuel ratio, whereby the secondary air supply control valve is controlled such that the secondary air is supplied to the engine exhaust gases at a continuously controlled, variable rate.