PURPOSE: To provide the joint structure of board material, which is simple and the joint area of which is neat and can be prevented from distorting due to secular change by a method wherein flat plate-like unit boards, each of which is made by pasting a plurality of sheets of board material together, are joined together without constructing complicated joint structure.

CONSTITUTION: The joint structure of board materials concerned is characterized by obliquely cutting off the ends of respective board materials 1, 1' and 1'', of all of which unit board is composed, by angles, which are changed in direction alternately or by every group consisting of a plurality of sheets of the board materials and by joining unit boards A and A' adjacent to each other together under the condition that their obliquely cut-off cutting areas 5 are engaged with each other. Further, in the joint structure of board materials concerned, the cutting-off angle of the obliquely cut off cutting areas 5 preferably ranges from about 30° to 60°.