To develop a manufacturing method capable of integrally molding a novel board not closing voids in the bonding of a charcoal material and a phenol resin and putting chaff and straw to practical use.

The powdery solid phenol resin are mixed with 2 pts. of the charcoal material with a particle size of 4-10 mm in an amount of 3-10 pts. to form a core material mixture while the powdery phenol resin is mixed with the cut matter of chaff or straw in a range of 8-12 pts. to form a surface material mixture. A mold assuming the shape of the board is filled with the core material mixture and the surface material mixture so that the core material mixture becomes an inner layer and the surface material mixture becomes upper and lower layers. In a hot press machine, the integrated board is obtained through a process (a) urging a temperature rise under pressure, a process (b) for stopping the temperature rise to hold a temperature to about 100°C and bringing about the softening of the phenol resin to bring about a structural change in the core material mixture and the surface material mixture by compression and a process (c) for again starting a temperature rise to urge curing reaction by the molecular crosslinking of the phenol resin.