PURPOSE: To attain miniaturization and low cast by using a drive device so as to gate an image signal for main scanning at an interval proportional to a moving speed in moving an optical system thereby eliminating the need for a driving device such as a motor.

CONSTITUTION: The rays of light from a light source 2 are irradiated on a paper sheet 1 on which an image such as a graph or a character to be read are written, the rays of light are received by a sensor 3 having photodetecting windows arranged in a line, the output of the sensor 3 is converted into a digital image signal 11 by a threshold value circuit 5 and a shaping circuit or the like, a turning angle detecting circuit 6 measures the amount of turning of a turning device 4 turned as an optical system moves and outputs a trigger signal 12 at every prescribed angle of turning. The trigger signal 12 generates a sampling gate signal 14 synchronizing with the period signal 10 of the main scanning, the image signal 11 is fetched only when the signal 14 is logic 1 so as to fetch the image 15 on the paper at a prescribed interval.