PURPOSE: To widen the filtration area in a device for passing raw water through granular filter media layers and filtering said water by packing the filter media between vertical supporting beds for the filter media.

CONSTITUTION: Supporting frames 16, 18 for granular filter media are provided opposite to each other in a filter tank 14, and granular filter media L, S are packed therebetween, whereby filter layers 15 are supported vertically. Raw water is fed into an inflow chamber 17 through a raw water pipe 24, and is passed horizontally through the layers 15, whereby the raw water is cleaned and filtered. The filter media L, S flowing out from the bottom end of the layers 15 are collected under the washing pipe 20. The media L, S collected under the pipe 20 are washed in the pipe 20, and the washed media L, S are carried to the upper part of the layers 15. The fine granular filter media S are concentrated on the outflow side and the coarse grain filter media L on the inflow side. The filtration area is increased and the raw water is filtered with good efficiency.