from multi vibrator 19 is applied to the and gate 21 through an inverter 20 the and gate 21 prevents any unexpected pulse f e from being transmitted from the collector terminal of the transistor 11 to the node 8 accordingly the unexpected signal f e as well as high frequency components of the ignition signal shown in fig7 are removed cut off or blocked so that a desired square wave shown in fig5 is obtained this square wave on the node 8 is applied to the measuring circuit 30 therefore the measuring circuit 30 can detect the true rotational speed of the engine accurately at any time without falsely measuring the unexpected pulse f e referring now to fig3 a circuit diagram of the measuring circuit 30 will be explained the node 8 is connected to an irq interrupt request terminal of a microcomputer 31 further a timer 32 is connected to the microcomputer 31 the timer 32 transmits the time data 34 rmt 34 to the microcomputer 31 in response to a request from the microcomputer 31 fig4 is a flowchart of an algorithm which is executed respectively in the microcomputer 31 whenever the rising edge of the signal from node 8 is applied to the irq terminal of the microcomputer 31 an irq routine so is executed at a step s1 actual time data 34 rmt 34 is read from the timer 32 and memorized.