Abstract: A method of manufacturing a thread-inserted window paper produces paper in which a thread is exposed to windows which are intermittently formed, and have clear contours, and in which thread is exposed surely at the window portions to make the thread clearly recognizable visually. The method is capable of being carried out with a relatively simple apparatus. Wet paper (1) in which a thread (T) is embedded in the paper layer is guided while being in intimate contact with a center roll (3) having protrusions (2) arranged on the surface thereof intermittently at a predetermined distance, the surface of the wet paper on the protrusions is frictionally rubbed by a friction roll (4) thereby moving fibers at the surface of the wet paper on the protrusions to form exposed portions of the thread intermittently on the surface of the wet paper and the wet paper is then dried by a customary method.