To use a standard catalyst, making a light-off characteristic excellent, by providing a detour path communicating an exhaust manifold and a manifold converter except a direct path and selectively using either one of the direct path and the detour path according to an operation state.

A manifold converter 1 is directly connected with an exhaust manifold 2 of 4-cylindered engine 8 and a detour path 4 communicating the exhaust manifold 2 and the manifold converter 1 is provided except direct paths 31, 32 formed on a direct portion of these exhaust manifold 2 and the manifold converter 1. The direct paths 31, 32 and the detour path 4 can be selectively closed by a switching valve part 5. This switching valve part 5 is constituted of check valves 51, 52 provided on each direct path 31, 32 and an opening and closing valve 53 provided in the vicinity of the other end of the detour circuit 4. By controlling the opening and closing valve 53 by a controller 10 according to an output of an exhaust gas temperature sensor 11, a light- off characteristic is improved.