To control the release of free formaldehyde, to reduce costs, and to prevent dimensional change caused by water and moisture absorption by using an isocyanate adhesive for addition to an adhesive of wood chips for a surface layer and a non-isocyanate adhesive to wood chips for a core layer.

The application amount of an isocyanate adhesive and non- isocyanate adhesive, based on absolutely dried wood chips, is made 4-10 wt.%, more preferably according to the kinds of adhesives, 5-9 wt.%. A wood chip layer 11 consisting of wood chips on which the isocyanate adhesive is sprayed is formed on a mat 10, a wood chip layer 12 consisting of wood chips on which the non-isocyanate adhesive is formed on the layer 11, and a wood chip layer 13 consisting of wood chips on which the isocyanate adhesive is sprayed is formed on the layer 12. As for the thicknesses of the wood chip layers 11-13, the layer 12 is 2-4 times as thick as the layer 13 which is as thick as the layer 11.