PURPOSE: To stably move a scanning optical device back and forth to scan by installing a means for detecting the actuated state of the scanning device when it is moved back, and setting an optimum control data based on detection information obtained by a preliminary scanning performed prior to the start of forming an image and then, controlling.

CONSTITUTION: When a motor 28 is rotated clockwise, scanning optical system holders 20 and 21 are moved to an original image scanning direction by a wire member 27 strethced in the scanning optical device 2, and the holders rapidly return a home position by rotating the motor counterclockwise. The actuated state of the scanning optical device 2 in moving back is detected by a front sensor and a back sensor installed on the front side of the home position at a prescribed distance. And prior to the start of forming the image, that is, at a warm-up time and an idling time, the detection information on the actuated state is inputted to a CPU for the image forming device, and the optimum control condition for the image forming device is set under the control by the CPU based on the detection information, and then, data on quantity of braking the motor 28 is set.