PURPOSE: To obtain a filter medium capable of being used a plurality of times by roughly disaggregating raw cotton, raw wool or the compression member of raw fibers made of them, removing dust and extremely short fibers while opening them, then, compressing the fiber member into a sheet shape and entangling the fibers with one another while washing them with high pressure water.

CONSTITUTION: A required medium is produced in accordance with respective steps of initially opening a fiber member S-1 obtained by the rough disaggregating of a raw fiber S a plurality of times (step K-1), removing dust and extremely short fibers D (step K-2) to obtain a fiber member S-2 sequentially arranged in a single fiber shape. Then, after the fiber member S-2 is compressed in a compressing step (K-3) to form it in a sheet shape, high pressure water is sprayed thereon to wash the fiber member and entangle fibers forming the fiber member S-2 with one another in a next step K-4. Thereafter, the sheet shaped fiber member S-3 is bleached (step K-5), the bleached sheet shaped fiber member S-3 is washed with water (step K-6) and dried (step K-7).