PURPOSE: To prevent the change of a comparig level caused by dust and temperature change by a method wherein the amount of light emission from a light emission element is controlled in such a way that a constant amount of light is supplied to a light reception element while no paper money is allowed to pass through a device.

CONSTITUTION: A light emission element 20 is set to produce the maximum amount of light when power is inputted. When a start switch 2 is depressed, the amount of light from a light emission element 18 is made to gradually decrease from the maximum value. When the amount inputted to a light reception element 22 becomes constant, the amount of light inputted to the light reception element 22 becomes also constant while no paper money is allowed to pass through, because SRFF11 is set. At the same time, a signal DAB making driving available is outputted so that SRFF34 is set to output a driving signal DRV. By so doing, the amount of light reaching the light reception element is made constant, while this prevent the variation of a paper money detecting level caused by dust and temperature change.