On embedding electronic watermark data in point group data obtained from a three-dimensional measurement, an x-y plane region defining the point group data is divided into a plurality of small regions so that a point group is produced with respect to each of small regions. The coordinate values of each point group are offset with making a barycenter of the point group be an origin point. A discrete Fourier transform is carried out in order to produce a Fourier coefficient sequence, which is modified into a watermarked Fourier coefficient sequence. The inverse discrete Fourier transform is carried out on the watermarked coefficient sequence in order to produce a watermarked complex number sequence. An optimum watermark embedding strength is calculated. On the basis of the embedding strength, the Fourier coefficient sequence is modified to produce a watermarked Fourier coefficient sequence which is is inversely offset into the watermarked point group data.