1. An image processing method for detecting alteration of data on an image formation medium on which a main data image and a copy detection latent image which becomes visible or more clearly distinguishable are formed, the method comprising: a first image creation step of creating an image in a state identical to a state of an image which is obtained by scanning the image formation medium; a one-way value operation step of effecting a one-way value operation on the image created at the first image creation step to calculate a one-way value; an image conversion step of converting the one-way value into an image representing the one-way value; a second image creation step of creating an image by superimposing the image representing the one-way value, the main data image, and the copy detection latent image.