PURPOSE: To reduce the height of a filter and unnecessitate any raw water pump, by combining a water-discharging siphon with a washing water tank provided at a low position while utilizing the fact that a sufficient washing effect can be obtained even when the pressure in a filtering tank is negative in a washing step.

CONSTITUTION: A filter medium 2 is provided in a hermetically sealed filtering tank 1, the water-discharging siphon 3 is provided on the upper side of the filter medium 2, and the lower end thereof is placed in a water-discharging pit 4 provided at a lower position. Raw water is supplied from a raw water tank 5 to a position on the upper side of the filter medium 2 through an inflow pipe 6, the washing water tank 7 is provided at a position lower than the tank 1 so that the pressure in the tank 1 becomes negative in the washing step, and a flap valve 8 is provided in the interior thereof. As raw water is passed downward through the filter medium 2 in a filtering step, suspensoids are trapped by the medium 2, the water level in the tank 1 is raised, and when the level reaches the top of the siphon 3, the siphon 3 starts to operate, thereby generating a water flow for a back washing step. In the case where the water level in the pit 4 is set to be lower than that in the tank 7 by a height corresponding to the loss head generated by the water flow, washing water flows naturally and the filter medium 2 exhibits a sufficient washing effect, even when the pressure in the tank 1 becomes negative.