1. A watermark detection method for detecting an embedded watermark in a detected object image, which watermark is imperceptible visually, the method comprising: a step for cutting a pixel block from an arbitrary position of said detected object image; a step for obtaining a coefficient matrix by performing a discrete Fourier transform on said block; a step for generating detected object component position information specified by a key; a step for multiplying a phase difference of a coefficient due to an amount of parallel displacement, with each item of said detected object component position information, and generating a position marker sequence; a step for extracting offset information which is the amount of parallel displacement when a start point of the embedded watermark is in agreement with a start point of the pixel block cut from the detected object image; and a step for detecting said embedded watermark from said pixel block cut from the detected object image on the basis of said offset information.