1. An image processing filter for correcting a pixel value of an image, comprising: a flatness calculation unit for calculating a flatness of distribution of pixel values of surrounding pixels positioned on a periphery of a target pixel to be filtered, and a pixel value composition unit for blending the pixel value of the target pixel and the pixel value of the surrounding pixel, based on a blend ratio corresponding to the flatness calculated by the flatness calculation unit, and generating a composite pixel value by combining the pixel value of the target pixel and the pixel value of the surrounding pixel.