To enhance the mechanical strength and to improve antimicrobial and mildewproofing performances by covering a spongy material with water- insoluble or hardly soluble polymer coating agent.

Spongy materials having konnyaku powder or glucomannan as the main materials are preferably used. At first, the konnyaku powder is added to water and, when it starts to swell, coagulating agent such as calcium hydroxide is added thereto, heated, and gelated. The konnyaku formed in such a way is frozen to separate water in the konnyaku and returned to the room temperature so as to provide a sponge. The surface of the sponge material is coated with polymer coating agent such as water-insoluble or hardly soluble ethyl cellulose. In this case, the coating process is so performed that the sponge material is dipped in coating solution formed by dissolving the coating material into appropriate solvent, taken out, and dried.