PURPOSE: To always obtain a stable color image by altering developing bias impressed on a developer holding member according to temperature information around a photosensitive body.

CONSTITUTION: A semiconductor laser 7 is made to emit light to irradiate the photosensitive body 5 with exposing light corresponding to the B signal of a negative, so that the electrostatic latent image of the B is formed. The photosensitive body 5 passes through a Y developing device 1, an M developing device 2 and a C developing device 3 which are in a non-developing state so as to reversely develop the electrostatic latent image on a developing roller 401 with a B developing device 4 in a developing state where the developing bias is impressed, then the toner image of the B is formed. After performing development, all the surface is irradiated with a destaticizing lamp 8 and the electrostatic latent image is erased by an AC corona charger 9. A thermistor 14 for detecting a temperature detects the temperature around the photosensitive body 5 and transmits output information to a developing bias controller. According to the output information, the developing bias impressed on the developing roller is changed. Thus, the stable color image can be always obtained.