An engine combustioning condition monitoring system monitors crankshaft angular position for detecting an engine cylinder in combustion cycle. The system derives variation of velocity of a crankshaft revolution on the basis of the monitored crankshaft angular positions and derives a combustioning coefficient for each engine cylinder. A value representative of the variation of the crankshaft revolution velocity is modified by the combustioning coefficient for deriving internal pressure in each engine cylinder. Discrimination of the engine combustioning conditon is made on the basis of the engine cylinder internal pressure.