PURPOSE: To obtain a desired copy image through a screen type image forming device by varying a bias voltage applied to a developing device according to whether line copying or gradation copying is performed.
CONSTITUTION: The development bias voltage is applied to the developing sleeve 11 of the developing device 7. Then, the application of an AC bias voltage 13 or grounding 14 is selected with a changeover switch 12. In this case, the application of the AC bias 13 forms the gradation copying and the grounding 14 forms the line copying. Thus, the development bias voltage is varied to select characteristics of the copy image to be obtained. Further, the nonimage part potential of a secondary image is varied during the development bias switching to prevent the fogging of a nonimage part. Thus, a desired image is obtained.
OOTSUKA YASUMASA
TANAKA KAZUMOTO
