PURPOSE: To obtain a detection signal of a high SN ratio that is free from any output fluctuation by providing a computing circuit which divides the sensor output by the ripple output and cancels the fluctuation of the sensor output.

CONSTITUTION: The detection output Z of a CCD which applies the pohotoelectric transformation to the light reflected on an original is inputted to one of two terminals of an analog computing element 10 consisting of a divider. Meanwhile the ripple output X received from an AC power supply is inputted to the other terminal of the element 10. Then the output Z serving as the sensor output and the output X are fetched by the element 10. The element 10 divides the output Z by the output X to obtain an arithmetic output Y=Z/X which includes no ripple component. The output Y is outputted to a sampling/holding circuit 21 to be held there for a fixed time and then undergoes the analog/digital conversion through an A/D converter 22. Then the digital signal is once stored in a memory 24 as the detection data.