A combustion state estimating apparatus for estimating the state of combustion in an internal combustion engine includes an angular acceleration calculator that calculates a crank angle acceleration, and a combustion state estimator that estimates the state of combustion in the internal combustion engine based on the crank angle acceleration in a crank angle interval in which an average value of inertia torque caused by a reciprocating inertia mass of the internal combustion engine is substantially zero. Thus, the combustion state estimating apparatus excludes the effect that the inertia torque caused by the reciprocating inertia mass has on the angular acceleration, and therefore is able to precisely estimate the state of combustion based on the angular acceleration.