The invention discloses an image forming device which has multiple first stage transfer units set parallel and a single power unit which simultaneously applies bias to the multiple first stage transfer units. When executing successive printing or both-sides printing and there is a need to change the bias value of the first stage transfer bias due to a mix of multiple types of recording media, thedevice is able to hold down electricity consumption, sustain image quality, and form images productively. When executing successive printing or both-sides printing, the application timing of the changed transfer bias due to the detection of a different recording medium to that of the initial medium is placed after the previous first stage transfer of a final color toner image is completed.