PURPOSE: To remove residual matters in a filter practically perfectly, by blowing air into the filter from its upper part, and backwashing the inside of the filter after lowering the water level in the filter.

CONSTITUTION: At the time of starting backwashing, backwashing water is supplied into a filter A from a gathering pipe 2 after closing an on-off valve 9. Succeedingly, compressed air is fed into the filter A from a supplying pipe 8 after the level of backwashing water in a outer shell 1 is raised to higher than through holes 4a, and the water level in the filter A is brought close to the holes 4a by adjusting an air pressure in an upper space of the filter A. As a result, the backwashing water which has passed through a filter medium 3 in the filter A enters a water discharging pipe 4 through the holes 4a, and at the same time, scum 7 floating on the water surface passes through the holes 4a together with the backwashing water to enter the pipe 4. Therefore, the scum 7 in the filter A is surely discharged to the outside.