1. An image forming method, comprising the steps of: i) inputting a medical image of an object, and ii) artificially forming a normal structure image, which has a shape and a texture that appear in an image of the object in a normal state; the step of artificially forming the normal structure image comprising the processes of: storing normal image data constituted by average shapes and average textures of objects in a normal state, statistically obtained from a plurality of medical images; setting a plurality of structure altering vectors to deform the average shapes and the average textures; and adding the structure altering vectors to the normal image data to generate normal image data that represents a normal structure image having a shape and a texture corresponding to the input medical image; the structure altering vectors being unique vectors which are obtained by analyzing the main components of the shapes and the textures of the plurality of medical images; and the structure altering vector setting process searching for parameters that result in the smallest different between the normal image data, to which each of the plurality of structure altering vectors multiplied by parameters are added, and the input medical image, to set the parameters and the structure altering vectors.