Abstract: An engine rotation speed detecting apparatus comprises circuitry for sensing the state of operation of an engine, circuitry for setting a plurality of respectively different masking periods of time, circuitry for detecting an ignition signal lasting for a period of time longer than any one of the plural masking periods of time thereby detecting an ignition period defined between adjacent ignition signals, and circuitry for computing the rotation speed of the engine on the basis of the detected ignition period, one of the plural masking periods of time being selected according to the result of sensing by the operation state sensing circuitry.