A charging device is designed in the following manner. Firstly, an optimization of the shape and size of an MC case is performed based on a film thickness and process speed of a photoreceptor (S1). Then, optimization of grid conditions (S2), saw-tooth conditions (S3), a distribution ratio of discharge current (S4) and a grid voltage (S5) are performed respectively, and a minimization of the discharge current is performed (S6). Lastly, surrounding conditions are taken into consideration (S7). The order of performing the processes in S1~S6 is not specified. By designing the charging device so as to have at least one feature obtained by the processes in S2-S7, a stable discharging operation, a uniform charging operation on the surface of the photoreceptor, reduction in amount of ozone generated when discharging, and reduced size and cost of the charging device can be achieved, and the charging device can be designed effectively in a short period of time.