A combustion monitoring apparatus for an internal combustion engine is provided which includes a voltage supply circuit, a combustion monitoring circuit, and a control circuit. The voltage supply circuit applies the voltage to plug electrodes of a spark plug mounted in a combustion chamber of the engine. The combustion ion monitoring circuit monitors a variation in amount of combustion ions existing between the plug electrodes to determine a combustion condition of the engine based on the current flowing through the plug electrodes produced by application of the voltage to the electrodes. The control circuit controls an operation of the voltage supply circuit so that the voltage is applied across the plug electrodes prior to a discharge duration of a spark plug mounted in the combustion chamber of the engine for monitoring preignition.