PURPOSE: To ensure the automatic level setting for the comparator circuit by setting the comparison level by the detection signal of the specified bit of the photo detector, thus obtaining the clear detection signals.

CONSTITUTION: The image sensor consists of photo detector row 1, CCD shift registers 2 and 3, comparator circuit 6 and others. Then the detection signal is extracted from the 5th bit of the utmost left edge in row 1 and then supplied to circuit 6 via amplifier circuit 4 and sample holding circuit 5. Thus comparison level Vc is set. In case the surface of the detected paper is pure white, the detection signal is high at 500mV and then 100mV at the black part each. Accordingly, border level Vc between the white and black levels is set at the position which is lower by 100mV than the white part of 500mV. Thus level Vc can be set in accordance with reduction of the definition on the surface of the detected paper. As a result, the comparison level can be set automatically for the image sensor.