To provide a filter having a simple and compact structure capable of filtering solid matter with a particle size as fine as 0.3 μ or less, which is contained in wide-ranging various fluids and being capable of automatically repeating filtration of a fluid to be treated and washing of a filter member alternately.

The filter comprises a plurality of ring-shaped filters layered as the main filter, by which a fluid flowing between the surfaces (facing each other) of adjacent ring filters is filtered. Electromagnetic valves for automatic opening and closing are each provided for a pipeline through which a treated fluid discharged via the main filter body is discharged and a pipeline for discharging the fluid containing solid matter separated by the above filtration. The electromagnetic valves are operated alternately, so that a filtration step of discharging a treated fluid and a step for washing the main filter while discharging a fluid containing solid matter separated via filtration are performed alternately.