PURPOSE: To measure an accurate engine speed of an engine from an ignition signal by restraining the effect of a timing retard, etc.

CONSTITUTION: The device in the title is provided with an ignition timing calculation means 50 for calculating the advance angle or delay angle of an ignition timing following the running state of an engine, a drive means 51 for sending out an ignition signal to an ignition means 52 following this ignition timing, the rotation speed of the engine calculation means 53 for calculating the rotation speed of the engine following the period of the ignition signal, a memory means 54 for renewing in order the calculated rotation speed of the engine and an ignition timing change judging means 55 for judging the change of the ignition timing when the ignition timing is compared with a prescribed value and the ignition timing exceeds the prescribed value. When this judging result is the change of the ignition timing, a previous rotation speed of the engine stored in the memory means is set as a present rotation speed of the engine, while when it is not so, a change over means 56 for setting the present rotation speed of the engine is provided.