To provide a development device that realizes, in a short time, a parameter design relating to control of the thickness of a developer layer formed on the surface of a developer supply roller, and corrects the thickness of the developer layer depending on deterioration with time and an environmental change, thereby keeping the image quality constant all the time, and to provide a development device design method.

The product of a ratio between the toner transport force F(Ra1) of a development roller 16 and the toner transport force F(Ra2) of a doctor roller 17, F(Ra2)/F(Ra1), and the circumferential speed ratio between the development roller 16 and the doctor roller 17, V2/V1, is a factor for determining the thickness of a layer of toner sticking to the development roller 16. Using the factor, the toner transport force (Ra1) and circumferential speed of the development roller 16 and the toner transport force (Ra2) and circumferential speed of the doctor roller 17 are set.