To provide a technology for extending further the range of operation condition capable of spontaneous ignition-combustion in an overhead-valve multi-cylinder engine capable of two cycle operation.

An index of combustion condition, which indicates a combustion condition of each combustion chamber, is computed on the change in internal pressure detected by a cylinder-pressure sensor 36. Computation is performed for a mean index of combustion conditions, that is, a mean value of the combustion-condition indexes in respective combustion chambers (S1). It is checked whether the index of the combustion condition in each combustion chamber falls within a prescribed allowable range in the vicinity of the mean index (S2, S3). If an index of combustion condition of a certain combustion chamber falls outside the range, a combustion-control parameter of that combustion chamber is adjusted, and thereby the combustion conditions of the respective chambers are approximated each other (S4-S5, S7-S8).