PURPOSE: To prevent poor operation of an aux. exhaust valve due to carbon attachment etc. by installing the valve on the aux. exhaust port side provided separately from a main exhaust port, and throttling the opening of the valve when the revolving speed exceeds a specified level, and actuating swinging when the acceleration/deceleration exceeds a certain value.

CONSTITUTION: A main exhaust port 1a and a pair of aux. exhaust ports 1b are provided at the peripheral wall of a cylinder 1, and the main exhaust port 1a is connected to the exhaust pipe, not illustrated, through a main exhaust passage 1c. The aux. exhaust port 1b is connected to this main exhaust passage 1c through aux. exhaust passages 1d, and an aux. exhaust valve 2 is interposed on each aux. exhaust passage 1d. This aux. exhaust valve 2 is controlled so as to restrict openings of the aux. exhaust passages 1d when the engine revolving speed exceeds a certain value, to restrict thereby rise of the engine output, and in case the acceleration/deceleration is over a certain level, the aux. exhaust valve 2 is swung to avoid poor operation of the aux. exhaust valve 2 caused by carbon attachment etc.