PURPOSE: To perform reliable decision of the combustion state of each cylinder throughout a whole operation region as probability and the statistical nature of a combustion fluctuation during lean burn operation and in turn to perform control of a combustion state, in a lean combustion type internal combustion engine.

CONSTITUTION: A rotation fluctuation control device, a combustion state deciding device, and a combustion state control device for an internal combustion engine comprise a crank angle signal period detecting means 404 to orderly detect the crank angle signal period of each cylinder based on a partial period during a time between a point of time when rush to a rotation angle region corresponding to a given crankshaft phase and a point of time when separation is effected; a rotation information detecting means 405 to orderly detect rotation information of each cylinder based on a detecting signal from the crank angle signal period detecting means 404: and a rotation fluctuation control means 406 to control each cylinder of an internal combustion engine 1 through detection of the rotation fluctuation of the internal combustion engine 1 based on a detecting signal from the rotation information detecting means 405. Constitution is made in such a manner that a partial period is detected at a plurality of the rotation angle regions of each cylinder, and the crank angle signal period of the corresponding cylinder is calculated based on at least one partial period of the partial periods of a plurality of rotation angle regions.