Abstract: A filter plate assembly in which a filter wall has a large number of small pyramidal projections on its opposite surfaces, a peripheral frame portion supporting the outer periphery of the filter wall and formed with a prefilt passage and a filtrate passage extending therethrough, and a filter medium supported by the peripheral frame portion and covering the opposite surfaces of the filter wall. The peripheral frame portion has a rigid frame and elastic frames sandwiching the rigid frame and joined thereto. Each of the elastic frames is provided, in its surface, with an open groove to form a prefilt inlet port through which the prefilt passage communicates with a prefilt chamber defined by the filter plate and another filter plate to be fitted therewith, whereby clogging of the prefilt inlet port is eliminated.