A filter maintenance monitoring method and a filter maintained thereby are provided, by which re-use of the filter by wash is smoothly and efficiently promoted. The filter includes that loaned from a filter owner to a filter user. The filter used for a predetermined purpose and having clogging of filter cloth by filtering dust in air is recovered to be washed for re-use. History data base stored in a storage portion of a computer is renewed based on wash history of the recovered filter. Then, re-use of the filter is decided based on the renewed history data base. Changes in the filter by damages or deterioration corresponding to the wash history are automatically grasped. Thereby, an efficient filter maintenance is realized.