1. A display driver converting input display image data so that a part of distribution of cumulative values of the number of pixels of each grayscale of the input display image data for one or plural frames is extended in a grayscale direction, and displaying the converted display image data on a display device, the display driver comprising: a conversion circuit which performs the conversion in accordance with a linear function when the grayscale of the display image data is a grayscale smaller than a specific grayscale and performs the conversion in accordance with a non-linear function when the grayscale of the display image data is equal to or higher than the specific grayscale.