1. An image processing method of inputting image data with registration signals embedded therein, subjected to geometric transformation, and extracting registration signals from said inputted image data to perform registration processing, comprising: a registration signal extracting step of extracting registration signals from said image data; a frequency property determining step of determining frequency properties of said image data; a geometric transformation identifying step of identifying geometric transformation to which said image data is subjected, using said extracted registration signals and determination results of said determination in the frequency property determining step; and a geometric transforming step of performing inverse transformation of said identified geometric transformation.