PURPOSE: To form an image with a nearly constant density even when toner in a developer is being consumed by providing a toner remaining quantity detecting sensor and a developing bias control means.

CONSTITUTION: A sensor 30 for nearly linearly detecting a toner remaining quantity in a developing unit 14 and a developing bias control means 31 for changing-controlling a developing bias according to a sensor output are provided. Accordingly, a developing bias voltage is changing-controlled according to a toner remaining quantity in the developing unit 14 so that, even when toner difficult to be electrified is remaining due to toner consumption, an appropriate developing bias for compensating the shortage of electrification is determined to be imparted to the developing device 14. Since a specified potential difference is formed between an image carrier 12 surface and the developing unit 14 so as to conduct sufficient toner attraction to the image carrier 12, image forming can be carried out with nearly constant density until toner is almost consumed. Consequently, low density (light) printing is prevented.