PURPOSE: To reduce a cost, to simplify the entire part of a filter device and to automatically operate the filter device by providing the filter device with a specific washing means for washing a filter member on site without interrupting a washing stage.

CONSTITUTION: A filter unit 1 consists of a casing 2, a wire net material filter member 3, a dash pot mechanism 4 for moving this filter member 3 and a rotary jet device 5 as a washing head which is freely movable with respect to the filter member 3 and injects a washing liquid under a pressure higher than the pressure of the fluid to be filtered so as to locally and concentrativelly wash the respective parts of the filter member 3. In the operation of the filter unit 1, the washing fluid is injected from the ejection port of the nozzle having the rotary jet device 5 toward the filter member 3 in the state of allowing the fluid to be filtered to be passed and is passed through the filter member 3, by which contaminants are removed from the filter member 3.