Anti-falsification paper in which a thread is exposed at window portions provided on the surface of the paper, and which does not produce any cockles even when wound up continuously in a roll and has good appearance in terms of design is provided. This anti-falsification prevention paper is combination paper having at least two paper layers including an outermost layer and the other layer. Window portions are formed in the outermost layer at intervals in a longitudinal direction of the paper. In the window portions there are formed watermarks of letters or pictures, and a thread inserted between the outermost layer and the other layer is exposed at these window portions. The ratio between the length (X) of the window portions in the longitudinal direction of the paper and the length (Y) in the same direction of non-window portions between the window portions is set at 3:1 to 1:2 to securely prevent the formation of cockles during the winding of paper. This setting of the length ratio of the window portions to the non-window portions also offers preferable effect on the design.