IMAGE FORMING DEVICE

PROBLEM TO BE SOLVED: To provide an image forming device that uses an exchangeable developing unit and prevents unnecessary consumption of developing powder at the start of image formation.SOLUTION: At the first printing after replacing a developing unit, the time X from when developing bias voltage starts to be input until when it reaches a predetermined value Vb0 is measured and stored. At the second printing, output of the developing bias voltage is started at the time Z when a time Xu passes from a charging start time t0, the time Xu being calculated by subtracting the time X from a predetermined time Y required for an end in a rotating direction (end of drum charging part) of a charging part of a photoreceptor drum to reach a developing position. The end of the drum charging part reaches the developing position at the same time as when the developing bias voltage reaches the predetermined value Vb0.