A false alteration detecting method using an electronic watermark method of fragile type by number theoretic transform. A processing section sets parameters P, N, .alpha. of number theoretic transform (S101), and reads original image blocks f.sub.i,j(x,y) (S103), conducts number theoretic transform of f.sub.i,j(x,y) to calculate the number theoretic transform blocks F.sub.i,j(x,y) (S105), determines the position (x',y') at which a signature image is to be embedded by using a randomizing function (S107), reads pixel values g.sub.i,j of the signature image for embedment from a storage section (S109), determines the embedment value .delta. of each block from the F.sub.i,j(x',y') of the embedment position and g.sub.i,j (S111), adds the embedment value .delta. to the F.sub.i,j(x,y) or subtracts the embedment value .delta. from the F.sub.i,j(x,y) to determine the number theoretic transform blocks H.sub.i,j(x,y) of the embedded image block (S113), calculates the inverse number theoretic transform of the H.sub.i,j(x,y), determines the embedded image blocks h.sub.i,j(x,y), stores them, and outputs them to an output section.