PURPOSE: To provide a filter apparatus for treating sewage, etc. wherein the clogging of small holes by filter media at the head section of nozzles is eliminated by jetting washing water from the washing nozzles, making use of natural dropping.

CONSTITUTION: A row of washing nozzles 6 for jetting high pressure water are arranged above a filter layer 2 in a filter apparatus, and a trough connecting to a pipe 5 for discharging washing waste water provided with a valve V3 is arranged above the row of the nozzles. First and second storage tanks 7, 8 for storaging washing water are arranged outside the apparatus 1, the tank 8 being provided at a position higher (with a head of H) than the washing nozzles 6. At back washing, a valve V5 is opened to feed high-pressure washing water from a pump PB to the lower part of the layer 2. At the same time, a valve V6 is opened to drop naturally washing water from the tank 8 provided at a position higher than the nozzles 6 to the nozzles 6 so as to jet washing water from the nozzles 6.