To provide an illegal copy preventing system which simplifies higher- order processing before mark detection to increase the processing speed, improve the reliability, and enrich applications.

In the copy preventing system, a pattern for copy prevention is constituted of a plurality of marks, and a reference mark and a reference pattern are preliminarily prepared, and copying is prevented when the detected mark and pattern don't coincide with the reference mark and the reference pattern. In the first stage, the pattern is detected and specified, and in the second stage, marks constituting the pattern obtained in the first stage are specified on the basis of their position coordinates, center coordinates, and directivities, and it is discriminated whether the detected pattern is correct or not in accordance with whether the specified marks coincide with the preliminarily prepared reference mark or not.