groove 4 so that the back part 3 b of the aperture extends both at the depth of the bottom 6 of the groove 4 and at the inner surface of the screen plate 1 past the lateral surface 7 b of the groove in the forward direction of the fiber flow the essential point about the invention is that on the side of the downstream side lateral surface 7 7 b of the groove there remains no threshold 8 a causing turbulence in the fiber flow and therefore impeding the passage of fibers as shown in fig1 which presents prior art screen structure solutions it is obvious to the person skilled in the art that the invention is not limited to the embodiment example described above but that it may be varied within the scope of the claims presented below thus the angles of the lateral surfaces of the grooves 4 and the positions and conicalness of the apertures 2 may differ from the above description the apertures may be e g completely cylindrical in which case they will be easier to produce than conical apertures it is also obvious to the skilled person that the downstream side back part 3 3 b of.