COLOR IMAGE FORMING DEVICE

PROBLEM TO BE SOLVED: To obtain a color image of high quality by executing control for stabilization at a toner concn. lower than a reference value, when absolute humidity in a device is high and making the toner concn. the same as the reference value and setting a developing bias higher, when the absolute humidity is low. SOLUTION: Developing conditions are controlled with temperature and humidity information obtained from a temperature and humidity detector 14 provided in the device. In other words, when the temp. and humidity detector 14 judges that environment is a low-temperature and -humidity one, a developing bias supplying means is set at -420V from -350V as a reference voltage, for instance, so that an image density is increased to secure a target image density. On the other hand, when the detector 14 judges that the environment is a high-temperature and -humidity one, a toner concentration control voltage is changed to obtain reduction from 3.5wt.% as the reference value of the toner concn. when temp. and humidity are normal to 2.75wt.% for instance, so that the increase of the image density is suppressed. Thus, the toner concn. or a developing bias voltage is controlled according to the environmental characteristic of a developer by an environmental change, to obtain high image quality with less fluctuations in image density.