1. An image conversion method comprising: a setting step of setting a value as a converted value corresponding to each of a plurality of classes, the value reflecting a mutual relationship between classes of pixel patterns each formed from a pixel classified as one of the plurality of classes and peripheral pixels; a selection step of sequentially selecting a pixel of interest from an input image; a classification step of classifying the selected pixel of interest as one of the plurality of classes in accordance with a neighboring pattern obtained based on a relationship between a value of the pixel of interest and values of peripheral pixels located at predetermined relative positions with respect to the pixel of interest; and a conversion step of converting the value of the pixel of interest into a converted value set for a class to which the pixel of interest has been classified in the classification step.