1. An electronic watermark embedding apparatus comprising: input image storage means for storing a pixel matrix G with n rows and m columns (2≦n and m) of an input image; singular value decomposition means for acquiring singular values s 1 to s r (s 1 >s 2 >. .. >s r ), left singular vectors u 1 to u r and right singular vectors v 1 to v r of the pixel matrix G read from the input image storage means; watermark information embedding means for creating a watermark information matrix W with r rows and r columns having a non-0 value as at least one nondiagonal element as electronic watermark information, creating a diagonal matrix S having singular values s 1 to s r as diagonal elements, and creating a matrix S w on the basis of S w =S+aW (a≠0); means for outputting the acquired singular values s 1 to s r , left singular vectors u 1 to u r and right singular vectors v 1 to v r as electronic watermark key information; and means for multiplying the created matrix S w by a matrix U having the acquired left singular vectors u 1 to u r as columns from the left and multiplying the transposed matrix of a matrix V having the acquired right singular vectors v 1 to v r as columns from the right to create a pixel matrix G w with n rows and m columns, and outputting it as an electronic-watermark-embedded image of the input image.