PURPOSE: To detect presence or absence of an origianl at a specified level, by providing a switching circuit switching a reference value by an on-signal of a luminous body disposed in the vicinity of a light receiving part and a comparing circuit comparing the output of the switching circuit and the output of an amplifying circuit.

CONSTITUTION: A switching circuit 7 switching a reference signal by an on-signal of a luminous body disposed in the vicinity of a light receiving material is provided. A comparing circuit 9 comparing a reference value switched from the circuit 7 with the output of an amplifying circuit 5 amplifying the output of a light receiving circuit 3 is provided. Resistances R4, R5, R6 of the switching circuit is set so as to switch a reference value in accordance with an on and off of the luminous body through the relation between voltage VCE1W3 across collector-emitter in an NPN transistor 6 and the on and off of the luminous body such as a fluorescent light for ascertaining the presence or absence of the original and reading the original. By such a way, in spite of the on and off of the luminous body, the presence or absence of the original is detected at a specified level.