PURPOSE: To enable the long-term use of a filter device by preventing clogging under an operation state and enhancing the durability of a filter.

CONSTITUTION: The vertically provided tank 1 of a filter device is divided into a primary chamber 3 and a secondary chamber 4 through a partition plate 2 having a through-hole provided to the central part thereof and a support is vertically provided so as to pass through the through-hole of the partition plate from the bottom plate of the secondary chamber 4 and a tubular filter is attached to the primary chamber 3 through the support and a raw water introducing pipe 6 is provided to the upper lateral position of the primary chamber 3. An automatic drain valve is provided to the lower lateral position of the primary chamber and a reflux lead-out pipe is provided to the secondary chamber 4. A raw water to be filtered flows in the primary chamber 3 from the introducing pipe and passes through the filter to enter the secondary chamber 4 from the through-hole of the partition plate and, when the raw water is refluxed to the original place from the lead-out pipe 7 provided to the lower part of the chamber 4, the automatic drain valve provided to the primary chamber is opened at a proper time for a definite period by the operation of a preset timer to allow the raw water in the primary chamber to partially flow out to the outside.