A combustion state detecting apparatus is provided for an engine having internal pressure sensor for detecting an internal pressure of a combustion chamber of the engine and crank angle detector for detecting a crank angle of the engine. The apparatus calculates a volume of the combustion chamber based on the crank angle detected by the crank angle detector and estimator for estimating a motoring pressure of the engine in accordance with a calculation equation using the calculated volume. The apparatus adjusts either of the pressure detected by the internal pressure sensor and the pressure estimated by the estimator during a compression stroke of the engine to minimize the error between the detected pressure and the estimated pressure. The apparatus determines a combustion state during a combustion stroke of the engine based on a relation between the pressure adjusted and the other pressure. The apparatus identifies parameters of the calculation equation during a compression stroke of the engine to minimize the error between the pressure detected by the internal pressure sensor and the pressure estimated by the estimator.