A filtering device includes a container, a cover attached on top of the container having a space communicating with an inlet opening and a chamber of the container, and a filter screen received in the container for filtering the fluid from the inlet opening to the chamber of the container. The filter screen includes an upper peripheral flange having a peripheral wall or a peripheral flap for making an excellent seal between the container and the cover. The space of the cover includes a changing volume for decreasing the flowing speed of the fluid.