1. An image processing device comprising: an edge strength calculation unit that calculates an edge strength of a pixel of interest in an input image from a stairs tone strength of the pixel of interest and a gradient of the pixel of interest, wherein the gradient is calculated from pixel values of the pixel of interest and pixels adjacent to the pixel of interest, and the stairs tone strength is indicative of differences between the gradient and a variance of the pixel of interest calculated from the pixel values of the pixel of interest and the adjacent pixels; and a mixing unit that calculates an illumination light component of the pixel of interest of the input image from the pixel value of the pixel of interest, an average value of the pixel values of the pixels adjacent to the pixel of interest, and a mixing ratio of the pixel of interest calculated from the edge strength.