To provide a copy forgery preventing paper making discrimination of a concealed pattern by the naked eye impossible and capable of certainly developing the concealed pattern at the time of duplication due to a copier to clear difference between original ticket paper and copied matter.

The copy forgery preventing paper is constituted of a background image 1 comprising a numerous-line pattern applied to the surface of a substrate material, and a latent image comprising a line group pattern formed to an arbitrary region of the numerous-line pattern so as to become a density almost same to that of the background image 1. The line group pattern of the latent image comprises gathering of two straight lines in the line width 1/2 the numerous lines of the background image 1, and the numerous lines of the background image 1 cross the straight lines of the latent image at a right angle.