PURPOSE: To detect with a high accuracy and exactly light and darkness by converting an image by an image forming lens of a reflected light from an object to be read, to an electric signal by an image sensor, and thereafter, inputting it to width detecting and background level detecting circuits, and quantizing compared information of both outputs.

CONSTITUTION: A postal matter P is illuminated through a condensing lens 3 by an illuminating lamp 2, and a reflected light is made to form an image on an image sensor 5 through an image forming lens 4. Also, an output of the sensor 5 is supplied to a width detecting circuit 8, and an output of the circuit 8 is supplied to background level detecting circuits 9a, 9b. Moreover, outputs of the detecting circuit 9a, 9b to which a prescribed DC bias is applied are supplied to one input + terminal of quantizing circuits 10a, 10b, and also an output of the sensor 5 is supplied to the other input - terminal.