PURPOSE: To prevent useless discharging of washing water and excessive washing by carrying out a countercurrent washing of filter medium only when the washing is necessary and stopping washing in other case.

CONSTITUTION: The filter medium on the lower part of filter bed in a vessel 3 is risen by using an air lift pipe 1 to transferred to a washing box 2. In the washing box 2, the filter medium is washed by bringing it into contact counter currently with the water of the vessel rising in the washing boxy 2 by using the head difference between the water level in the vessel and the discharged washing water level, and the washed filter medium is returned again to the filter bed. An air chamber is provided to the lower part of a float 8 in the washing box 2 to introduce air so that the floating force of the washing box 2 is adjustable. Consequently, the floating force is increased when the washing is not necessary, so that the height of discharge part of discharge washing water in the washing box is raised higher than the water level in the vessel.