PURPOSE: To shorten an image formation time and to reduce an image forming device in size on the whole by detecting a form and controlling the image formation time of the next form on the basis of the detection output.

CONSTITUTION: The form P conveyed by an aligning roller 11 is sent to an image transfer part 12. The fed form P is charged electrostatically by a charger 13 and a toner image on a photosensitive drum 2 is transferred. A switch 22 for detecting the paper feed state is provided near the aligning roller 11, and a switch 23 for detecting the paper discharge state is further provided near a paper discharge roller 19. The detection time of the form P is measured with the output of the switch 22 and the movement of a platen 3 is specified according to the measured time to control the image formation.