A method for embedding a watermark into digital data, when the watermark is to be embedded in a digital image, independently changes real number components and imaginary number components of each of coefficient values of a complex watermark coefficient matrix using key, from the watermark to be embedded in the digital image, a step for performing a discrete Fourier inverse transform on the sequence matrix of the changed watermark and generating a watermark pattern; and a step for adding like tiling the water mark pattern to the original image, and generating an embedded image.Further more, a watermark detection method for detecting a watermark from a digital data, a step for separating a block from an arbitrary position on the detected object image, a step for performing a discrete Fourier transform on the block and obtaining a sequence matrix, a step for generating position information for a component that is to be detected and that is specified by the key, a step for detecting a position marker sequence by calculating a phase difference of a sequence by an amount of parallel displacement, for each item of the position information, and extracting offset information which is the amount of parallel displacement when there is agreement between a start point of an embedded watermark and a start point of the block cut from the detected object image, and a step for detecting the embedded watermark cut from the detected object image.