To provide an image forming device which excellently performs the electrostatic attraction and conveyance of a transfer material even when an image carrier which is not used at the time of monochromatic image formation is separated from a conveyance rotary body and also excellently perform the electrostatic attraction and conveyance of the transfer material, specially, even when a conveyance path by the conveyance rotary body is arranged perpendicularly or obliquely.

When the transfer material P is electrostatically attracted and conveyed by a conveyor belt 3 at the time of monochromatic image formation, a transfer roller 4c among transfer rollers 4m, 4c, and 4y facing image formation units 1m, 1c, and 1y which are separated from the conveyor belt 3 and not used is applied with a transfer bias voltage having the same positive polarity with a plus transfer bias voltage applied to a transfer roller 4k facing an image formation unit 1k which performs image transfer operation nearby the conveyor belt 3 and the transfer rollers 4m and 4y are applied with a transfer bias voltage having the negative polarity.