To control the air-fuel ratio of exhaust gas led to flow into catalytic converter rhodium, approximately in a theoretical air-fuel ratio so as to obtain high exhaust gas purifying efficient by providing a secondary air control valve in a secondary air introducing passage, and interlocking and connecting the secondary air control valve to the throttle valve of a carburetor.

A catalyst device 4 consists of catalytic converter rhodium and arranged in an exhaust passage 3, and a secondary air introducing passage 5 is connected thereto through a check valve 6 by which the upstream side of the catalyst device 4 and an air cleaner 2 are opened only on an exhaust passage side. A secondary air control valve 7 is arranged on the way of the air introducing passage 5, and interlocked and connected to the throttle valve 9 of a carburetor through a link 8. Hereby, when an engine load is small, the openings of the throttle valve 9 and the secondary air control valve 7 are diminished so that a secondary air supply amount is diminished. While, when the engine load is increased, the secondary air supply amount is increased so that the air-fuel ratio of exhaust gas led to flow into the catalytic converter rhodium can be controlled in a theoretical air-fuel ratio.