PURPOSE: To store information deciding the adjustment processing starting timing of optical beam before starting the next rotating polygon mirror by storing the starting end state to the required speed in the prescribed time just before starting the rotating polygon mirror.

CONSTITUTION: When the driving of the rotating polygon mirror is successively directed based on the presence or absence of the printing request to be inputted from an external device (host computer 14) from an indicating means (print control part 16), starting end decision information to the required speed obtained by monitoring the rotation speed of the rotating polygon mirror for the prescribed time from the driving starting time is written in a storage means (scanner ready storage means 30). The decision information to decide the light quantity adjustment starting indication timing in the next print sequence is renewably stored in succession.