PURPOSE: To make a simply configured circuit capable of detecting that a reference cylinder is operated on a specific stroke by counting the number of revolution pulses which are developed at the presence of cylinder pulses by means of a counter, comparing the number with a specified value and generating a cylinder identifying signal.

CONSTITUTION: A revolution pulse of a crank angle sensor 1 is inputted into an NAND circuit 2 via a voltage level conversion circuit which consists of a resistance R1 and a Zener diode ZD1, while the cylinder pulses of the sensor 1 are inputted into the NAND circuit 2 and a reset circuit 3 via a voltage level conversion circuit which consists of a resistance R2 and a Zener diode ZD2. The cylinder pulses are generated when respective cylinders are operated on specific strokes, and since the pulse width of a reference cylinder is longer than those of the other cylinders, the number of revolution pulses which is counted by a counter 6 exceed a specified value as for the reference cylinder, thereby generating a cylinder identifying signal. The simply configured circuit, as such, may detect that the reference cylinder is operated on a specific stroke.