First moss pieces (3a) each having a larger size and second moss pieces (3b) each having a smaller size are prepared by cutting raw material moss. Firstly, the first moss pieces (3a) are sprayed onto the surface of a panel material (1) on which an adhesive agent (2) that can be expanded and cured upon the absorption of water has been applied, thereby adhering the first moss pieces (3a) onto the panel material. Subsequently, the second moss pieces (3b) are sprayed onto the surface of the panel material (1), thereby weaving the second moss pieces (3b) into gaps (4) between the first moss pieces (3a).
