PURPOSE: To prevent the outflow of move than a necessary amount of a liquid to be filtered and to reduce a washing time by connecting one end of a return pipe having an inner diameter smaller than that of a siphon tube to the highest part of said siphon pipe and opening the other end thereof at a position under the overflow port of a storage chamber and providing a washing time controller to a control tank.

CONSTITUTION: When a filter material 8 is washed, the liquid level of the liquid to be filtered in a storage chamber 9 as the liquid to be filtered after washing flows out toward a control tank 3 through a siphon pipe 12 and, when said liquid level reaches the opening end of the return pipe 18 opened to the storage tank 9 or less, the open air invades in the return pipe 18 and, as a result, the siphoning action of the siphon pipe 12 to which the return pipe 12 is connected is stopped and the outflow of the liquid to be filtered to the control tank 3 through the siphon pipe 12 is also stopped to complete the washing of the filter material 8. The washing time from the point of time when a liquid feed pump 5 is stopped to the restarting time of said pump 5 is arbitrarily controlled by a washing time controller 30 equipped with a volume controller 20a and a flow rate control valve 20b. By this method, the consumption of the filtered liquid and the washing time can be reduced and the operation of the apparatus can be made efficient and economical.