US Patent 7395935 Filtering device

A filter device is provided with a filter tank, a filter drum disposed in the filter tank, and a drive mechanism which rotates the filter drum. The filter element is composed of a porous plate that is formed with a large number of circulation holes. The porous plate is formed into a cylindrical shape. An inlet port opens in an outer peripheral surface of the porous plate. An outlet port opens in an inner peripheral surface of the porous plate. The circulation hole is formed by etching. A smallest opening of the circulation hole is formed between the inlet port and an intermediate part with respect to a plate thickness direction. An opening width of the circulation hole increases from the smallest opening toward the outlet port to form a tapered shape.