In a first image forming method, a medical image of an object is inputted, and a normal structure image, which corresponds to the inputted medical image and represents a normal structure of the object, is artificially formed. In a second image forming method, a normal image signal corresponding to a normal image, which is a medical image representing a normal structure of an object, is acquired, a plurality of structure altering vectors for altering the structure of the object are set, and an addition process for adding the plurality of the structure altering vectors to the normal image signal is performed, and a normal structure image signal, which represents a normal structure image of the object, is formed.