PURPOSE: To discriminate the number of paper sheets accurately for a long period of time without the influence of the change in the output characteristics of transmitted light signals that arises on account of secular changes, etc. by changing the threshold level at all times on the basis of the transmitted light signal when there are no paper sheet.

CONSTITUTION: The signal [voltage at the point (c)] which is decreased in the level of the transmitted light corresponding to the quantity of transmitted light when there are no postcards at all is stored by a capacitor 59. The magnitude of the signal can be regulated by regulating a variable resistor 23. Therefore, the threshold level can be set at the intermediate between the voltage at the point (c) corresponding to the light transmitted through one sheet of the postcard and the voltage at the point (c) corresponding to the light transmitted through two sheets. Even if the output characteristics of the transmitted light signal changes on account of a secular change or the like later, the changes appear at an equal ratio in the cases when there are no post cards, there is one sheet and there are two sheets; therefore, the threshold level can be maintained at the intermediate of the voltage at the point (c) when there is one sheet of the postcard and the voltage at the point (c) when there are two sheets by changing said level at all times. Thus, ≥2 sheets of the postcards are detected accurately at all times for a long period of time.