PURPOSE: To efficiently purify exhaust gas by using a low-priced carburetor without using an expensive oxygen-concentration detector and an expensive electronic control unit for controlling an air-fuel ratio.

CONSTITUTION: A variable venturi type carburetor 2 is installed onto an intake manifold 1, and a catalytic converter 4 is installed on the downstream side of an exhaust manifold 3. A valve port 36 whose opening area is controlled by a linear solenoid valve 40 is installed into an air bleed passage 34. A solenoid driving circuit 50 is controlled by a water-temperature switch 58 in correspondence with the temperature of engine cooling water and a temperature sensing switch 59 installed onto the ternary catalysis converter 4. That is, when the engine temperature is above a predetermined value and the temperature of the catalytic converter is below a predetermined value, the valve body 41 of the valve port 36 is kept at a constant opening degree, and the lean mixed gas is supplied into a cylinder, and the catalytic converter is heated and activated.