PURPOSE: To detect the zero-cross point of a rotation signal by a magnetic pickup type sensor with high accuracy by changing a detected voltage (slice level) depending on the rotation signal and the magnitude of its peak voltage, and setting the slice level at the time when hysteresis operates, in a specific range.

CONSTITUTION: With a rotation signal detecting circuit 3, the signal level of a rotation signal 2a outputted from an engine-rotation detecting machine 2 is voltage-divided by respective resistors 7 to 9, and is inputted to a minus input terminal of a comparator 21. On the other hand, the slice level 23a which is voltage-divided by resistors 16, 17 and determined by the voltage biased by resistor 18, is inputted to a plus terminal of the comparator 21. Then, the inputs of the respective terminals are compared with each other. As a result, when a voltage-divided signal of the rotation signal 2a is, for example, higher than the slice level 23a, the output of the comparator 21 is reduced to a low level. Then, when the output of the comparator 21 is changed, a comparation voltage is changed, and hysteresis is given to the reference voltage. By setting the comparation voltage in the range 5 to 10mV, malfunction can be prevented, and the detecting accuracy can also be improved.