A main controlling part is configured to choose and execute either a constant belt speed control which controls a driving motor (a driving source for an intermediate transfer belt) to move the intermediate transfer belt at a predetermined target belt speed or a constant motor speed control which rotates the driving motor at a predetermined target rotational speed according to a print command, and then, prior to a first-time print job, after executing a writing position correcting process while moving the intermediate transfer belt under the constant belt speed control, forms a color shift detecting image by switching from the constant belt speed control to the constant motor speed control, measures a difference between amounts of color shift, and executes a speed correcting process for correcting the target rotational speed of the driving motor in the constant motor speed control based on the measurement result.