PURPOSE: To provide a filter apparatus for treating city sewage, etc. wherein the filter media-washing efficiency is improved and the filter media layer may be washed to a deep site by jetting air-containing washing water through nozzles in rotary blades.

CONSTITUTION: An original water-feed pipe 11 is connected to a filter apparatus at a position above a filter media layer 2, and a filtrate-feed pipe 12 and back-washing water feed pipe 13 are connected threrto at a position below the layer 2 provided in a filter apparatus 1. Two stages of rotary blades 4a, 4b are connected at above the layer 2 to a pipe 5 for feeding high-pressure washing water so as to turn freely by means of rotary joints 6. Numbers of nozzles 7a, 4b are provided in the blades 4a, 4b each in opposite directions against the center of rotation. The upper blade 4a is fixed behind the lower blade 4b at an acute angle α in the direction of rotation. The blade 4b first washed the filter media, and then the blade 4a washes further the filter media.