PURPOSE: To detect accurately a mark without requiring adjustment by differentiating a sensor output signal, identifying the level of a differentiated signal, and removing noise components.

CONSTITUTION: A differentiator 5 differentiates a sensor output signal a', then a differential signal b' is obtained. A peak detector 6 obtains a peak detection signal d' from a comparator 8. While comparators 9 and 10 slices a signal b' with slice levels (+TH) and (-TH), and generates output signals e' and f'. These signals e' and f' are fed to an NOR circuit 15 in a shaping circuit 7 to form a set signal g'. While a signal d' is fed to a latch circuit 14 and sets the circuit 14 to a condition in correspondence with the level of the signal d' when the signal g' is turned on. The circuit 14 holds the condition directly before an off period when the signal g' is in the off period, which makes noise fail to actuate the circuit 14 ignoring the noise, thus a correctly shaped signal h' is obtained from the circuit 14.