ratio u03b8 e depending on the engine speed n e and the throttle valve opening ratio u03b8 th and an ignition timing control map of fig7 which determines an ignition timing u03b8 ig of the ignition plug 22 depending on the engine speed n e and the throttle valve opening ratio u03b8 th the cpu 40 transmits a drive signal u03b4 u03b8 e to the exhaust control servo motor 30 to set the exhaust opening ratio u03b8 e obtained by the exhaust opening control map and at the same time allows the ignition plug 22 to be ignited with the ignition timing u03b8 ig obtained by the ignition timing control map the exhaust opening ratio u03b8 e in the exhaust opening control map in fig6 is determined such that the gas filled in the cylinder is self fired with a timing most desirable for operation of the spark ignition type two cycle engine 1 in the map 6 an exhaust passage opening ratio u03b8 e0 between the minimum throttle valve opening ratio u03b8 th0 and an opening ratio u03b8 th1 slightly larger than the minimum throttle opening ratio u03b8 th0 is set to be larger than an exhaust passage opening ratio u03b8 e2 of a throttle valve opening ratio u03b8 th2 larger than the opening ratio u03b8 th1 in the case where the load cell 39 detects the generation of abnormal combustion in the normal combustion operational area the cpu 40 reduces the exhaust opening ratio u03b8 e by the positive correction and at the same time it delays the ignition timing u03b8 ig and in the ar combustion operational area.