low level input pulse even after time t2 elapses in d2 fig5 d output e is changed to low level after time t2 has elapsed detection of the leading edge of the sheet medium is accomplished by the aforesaid output change from high to low level the pulse monitoring time t2 is determined by the permissible error in leading edge detection as specified by secondary scanning speed diameter and rotation period of each roller the requirements for precision sheet leading edge detection are satisfied in the present embodiment if time t2 is 20 ms or less although the intermittent illumination period is likewise 20 ms or less 50 hz or greater based on time t2 permissible values it is set at 100 hz or greater detection error of output e does not occur even if a single pulse in fig5 d is missed due to surface conditions of the metal core 12a of the small diameter roller since time t2 is greater than a double of t1 to prevent detection errors due to synchronization of the detection pulse with soiling or damage on the surface of the small diameter roller 11a or metal core 12a the oscillation cycle of oscillator 18 is set so as to avoid integer multiples of the rotation period of the small diameter roller in the present embodiment oscillation frequency is set at 100 hz t1 5 ms illumination rate is 25 and pulse monitoring time t2 is set at 10 ms because the period of the small diameter rollers is 1 1 to 1 4 hz although the sheet detection apparatus is shown having the reflection type photosensor 13a mounted opposite metal core 12a of small diameter roller the smaller the reflecting surface area opposite sensor.