a light beam is cyclically deflected by a rotating light deflector to scan an object such as an information storage medium . the scanning beam is detected by a light sensor located outside of the scanning region for synchronizing primary scanning . a plurality of n clock signals c 1 through c n are successively generated by the delay element from an image scanning clock signal and a reference clock signal having the same frequency as that of the image scanning clock signal . one of the clock signals c 1 through c n is selected as the image scanning clock signal dependent on the output signal from the light sensor . dependent on the tap - to - tap delay error of the delay element : ± α %, the maximum error of an image scanning position : 1 / n pixel , and the period of the image scanning clock signal : t 0 , δt 0 which meets a prescribed condition is selected as a tap - to - tap delay time of the delay element .