To provide a method for treating an air filter by which a filter medium of the air filter can be easily and surely cleaned and used excellently over a long period of time while reducing the environmental load.

The filter medium of the air filter is treated with a fatty acid and used. The used filter medium is cleaned by using an alkali. Otherwise, the filter medium is treated with a fatty acid salt and used. The used filter medium is washed with water. According to this method the dust consisting of organic matter can be stuck to the fatty acid or the fatty acid salt and the dust-stuck fatty acid or the dust-stuck fatty acid salt can surely be washed away by the surface-active action of the fatty acid or the fatty acid salt when washed. Thus, if the filter medium of the air filter is treated and washed periodically by using this method, the filter medium can be used excellently over a long period of time, the maintenance cost of the air filter can be reduced and the environmental load can be reduced by reducing industrial waste.