It is produced in accordance with in-canal installation for the purpose of fine screening of waste water. One of the two screen sets in the step screen is constant and the other is mobile. The particles coming with the flow are held on the surface of the screen and carried to the discharge point by moving the moving screen that is sets upwards step by step. Since the screening process continues during the uptake of the particles, the removal of the finer particles is performed thanks to the accumulated particles form that provide a filter structure on the surface of the screen.