To provide a method for filtering water in e.g. a cistern by which the water can be passed through the nearly entire area of a filter medium without fail to substantiate an increase in a filtration area, a good water environment is kept for a long term, the appearance is beautiful, and care such as the exchange or cleaning of a filter medium can be simply done and to provide a filter therefor.

This method is one for filtering water in a cistern 1 or the like, wherein a filtration part 3 is laid on the bottom 2 of the cistern 1 or the like. A water passage 6 which watertightly separates water from the part 3 is open in part 5, contains no filter medium on the delivery side and is laid over the substantially entire width of the laid filter medium on the filter medium of the part 3. The filtered water from the filter media is delivered or recirculated by a difference in liquid level or by force by a pump 3. The filter medium is detachably integrated with a separation means 4. The filter medium of the filtration means is a combination of a coarse filter medium with a fine filter medium, a granular filter medium formed by laying a coarse filter medium over the nearly entire surface of the cistern or the like, laying a granular filter medium over it, and further laying a fine filter medium over that, or a granular filter material being a mixture of pebbles with sand or the like, and a synthetic resin plate 5 such as polyvinyl chloride one is laid on such filter medium. A filter therefor is also provided.