PURPOSE: To reinforce a board in its slant direction and to eliminate joint line on the board surface by making a belt shaped material into a cylindrical tube in helically winding and then making this flat to join the inner surfaces thereof to each other, and by joining another board material to at least the one surface of the board material obtained.

CONSTITUTION: The belt-shaped material (1A) is wound helically so as to be a cylindrical pipe (2A). In this example, a duplex tube is made because the belt shaped material is superposed at each halve and then the duplex tube is made flat. An assembly of three bending rollers, preferably a diameter defining roller are arranged so as to mold the belt-shaped material (1A) to be a helical shape, and it is preferable that the process for making the cylindrical tube to be a flat may be employed in compression by for example an assembly of press instruments. At this time, any methods should be taken whether a material with joining property is inserted into the inner part, an adhesive is applied on the inner surface or the flat material is raised in its temperature. In the case where the composite board material is worked into the board material without any joint lines, another board material without joint lines should be joined to the board surface. By this constitution isotropic property can be improved and the composite board material reinforced in its slant direction can be obtained from the narrow and the few number of an original board.