PURPOSE: To continuously execute a process of forming two toner images and a process of transferring the images to two sides of the transfer material.

CONSTITUTION: A unit IA and a unit IB for storing a photoreceptor belt 1, plural developing devices 2Y to 2K, an electrostatic charger 3 and an optical writing device 4, etc., with a symmetrical layout are arranged on both sides of the paper feeding surface from the upstream side of a transfer material feeding direction at a prescribed distance, the toner image formed by the unit IA is transferred to one side of the fed transfer material, and then, the toner image formed by the unit IB is continuously transferred to the other side of the transfer material so as to record the images on both sides.