PURPOSE: To invariably assure the formation of a high-quality image regardless of the change of the image quantity to be formed.

CONSTITUTION: In an image forming device arranged with multiple image forming units 10 each provided with image forming sections 11, 13,... and a heat fixing section in the sheet conveyance direction, the transfer unit power supply 19B of the downstream side image forming unit 10B can be switched to voltage V1, V2, V3, and a transfer voltage switching control means switching the transfer voltage together with the power supply 19B is provided. The transfer voltage switching control means is constituted of a corresponding data memory means 22, transfer voltage specifying means 21, 22, and switching signal generating means 21, 22 to allow fully automatic switching.