PURPOSE: To correct a skew without moving a sheet by providing a control means for outputting the control signal of an optical system and the drive signal of an optical system regulating motor after receiving the detection signal from a detecting means followed by correction.

CONSTITUTION: A detecting means 5 for detecting the state of a carried sheet and a control means 4 for receiving the detection signal from the detecting means 5 and outputting the control signal CS of an optical system and the drive signal MS of an optical system regulating motor CM are provided between a waiting roller for carrying a sheet P and a sensitizing drum 2. An offset amount X is detected from the detection signal by the detecting means 5 to correct printing start position and printing start timing, the skew angle G of the sheet is also calculated from the detection signal, and the position of an optical system 3 is regulated by this drive signal MS to correct the skew angle.