To reduce granular noise corresponding to a minimum display level generated when gradations of a PDP(Plasma Display Panel) is increased by error diffusion processing.

When analog video signals for two adjacent pixels A, B of a PDP are inputted, each of them is converted into a digital data and this digital data is processed by an inverse γ-correction, and the data processed by the inverse γ-correction is processed by error diffusion, to generate a minimum display level value. Then, prescribed values based on the generated minimum display level values for the pixels A, B are distributed to the digital data for the pixels A, B processed by the inverse γ-correction, and the pixel is replaced with the pixel B for each line or each field.