Abstract: A multi-layer filter, wherein a filter medium inside a rotatable or swingable horizontal drum (3) is formed in a multi-layer shape with a single or a combined grading, specific gravity, and characteristic of filter medium desirably according to the purpose of treatment and washed with raw water while rubbing the filter medium by rorating or swinging the drum (3) so that contaminent can be removed to surely recover the filter medium at each layer, a raw water inflow tube (5) and a filtered water outflow tube (7) are formed in a shape of double-pipe, a sandproof means and a filter medium agitating means are installed on both or either of the tubes, and a cleaning means and an adhesion prevention means for preventing seaweed from adhering to the inside wall of the drum (3) are installed between a filter net (15) and the inside wall of the drum (3), whereby a structure can be simplified, controllability and maintainability can be facilitated, production cost and maintenance cost can be reduced, and durability and

Abstract: A filter medium comprising first and second filter mediums joined together. The first filter medium provides relatively large voids and has high porosity, while the second filter medium provides relatively small voids and has low porosity. Raw water is mainly introduced into the first filter medium, and water branching therefrom flows through the second filter medium, at which biological filtering is conducted by aerobic microorganisms breeding therein.