In an apparatus and method for estimating a stability factor of combustions in a vehicular internal combustion engine, a engine revolution synchronized filter processing is carried out for a signal derived in synchronization with the engine revolutions and, thereafter, a time synchronized filter processing is carried out to extract the frequency component correlated to the engine combustion stability factor. Noise components synchronized with the engine revolutions, based on a revolution speed sensor working accuracy error, and based on tire (road) wheel revolution (rotation) first-order vibration frequency are eliminated. In addition, a resonance frequency component in a vehicular drive train is extracted as a frequency component correlated to the combustion stability factor.