Detection of engine rotation speed in spark ignition internal combustion engine

A spark ignition internal combustion engine (2) performs ignition within a fixed ignition crank angle range. The operation control device comprises a programmable controller (1) and a unit crank angle sensor (9) outputting a unit crank angle signal on each unit crank angle. The controller (1) calculates the engine rotation speed based on the unit crank angle signals (S1). By preventing the calculation of the engine rotation speed based on the unit crank angle signals detected in the ignition crank angle range, errors in the calculation of the engine rotation speed resulting from engine ignition noise are eliminated and a precise engine rotation speed is obtained.