To provide a maintenance method capable of prolonging an exchange cycle of a fine filter to the maximum by prolonging a service life of a secondary filter, simplifying cleaning and regeneration of a primary filter, reducing costs of filter media and the maintenance, and utilizing the secondary filter as far as the limit of the service life.

Relating to the filter device using the primary filter A and the secondary filter B combined in series, the primary filter can alternatively perform a filter process and a back-washing process, and not only cleaned liquid discharged from the primary filter during the filter process, but also contamination liquid discharged during the back-washing process are caused to flow out to the secondary filter and the fine filter is exchangably arranged in the secondary filter. In the maintenance method, when the fine filter of the secondary filter has been served as far as the nominal service life, the primary filter is switched to the back-washing process from the filter process, and impurities collected on the primary filter are back-washed and are collected by the secondary filter, and the fine filter is exchanged to a new fine filter.