An image processing method for detecting alteration of data formed on an image formation medium on which a main data image and a copy detection latent image are formed. The method comprises 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 on which the main data image and the copy detection latent image are formed, 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.