To enable mineral ions to be eluted for a long period by constituting a filter medium in which ionic crystal containing mineral ions is held.

The filter medium 3 is constituted of material in which ionic crystal containing mineral ions ion held in the inside of a solid cellulose-based polymer dotted with holes capable of penetrating water and a mineral component. When water is passed through the filter medium 3, water is penetrated to the inside of the filter medium 3. Ionic crystal is ionized by this water to form mineral ions. The mineral ions are moved along the holes of the gelled cellulose-based polymer while being hindered by the cellulose-based polymer and eluted into water passed on the surface of the filter medium 3.