PURPOSE: To prevent malfunction of sensing devices by furnishing a photo quan tity comparator, a hold circuit and a timing control circuit.

CONSTITUTION: This arrangement includes a photo quantity comparator 6 to control the amount of light emission through comparison of the light received by photoelectric sensors 1, 2 with a given threshold value, a hold circuit 7 to hold the light emission quantity voltage controlled through said comparator 6, and a timing control circuit 8 to make switching from the comparator 6 to the hold circuit 7 or vice versa according to the condition of sheets being transported. Among them, the comparator 6 senses rise of the ambient temp., secular change in the light emitting part 1, and drop of the amount of light reception due to influence of dust etc. attached to the light emitting part 1 and receiving part 2, and also raises the supply voltage to the light emitting part 1 so as to accelerate light emission. This can prevent malfunction of the sensing device.