PURPOSE: To obtain an image without unevenness by adjusting voltage for electrostatic charging and developing bias voltage and forming a halftone image or a solid image.

CONSTITUTION: When a 1st mode is selected, ordinary image forming operation is executed and an electrostatic latent image formed on the surface of a photoreceptor drum 28 is developed with toner. Meanwhile, when a 2nd mode is selected, halftone image forming operation is executed. Namely, the surface of the drum 28 is uniformly electrostatically charged by an electrostatic charger 32, and a toner image is formed by a potential difference between the surface of the drum 28 and a developing device 36. At such a time, by adjusting the voltage for electrostatic charging and the developing bias voltage, the potential difference between the surface of the drum 28 and the developing device 36 is adjusted and image density is adjusted. When the image density is made thick, the solid image is formed.