To provide a single plate structure requiring no adhesive, eliminating necessity of a separate jointing step, having a sufficient strength as an interior material, material for furniture or the like, and being strongly jointed, and also provide a compact joint structure of a plate material making it possible to make effective use of a light and soft material such as a cedar, abies sachalinensis or fir, or a material that could not be used due to the size irregularity.

A plurality of plate members 21, 22 are jointed together at their edges 25, 26, and are formed integrally as a single plate U with the entire plate including a joint part J1 compacted by compression molding. The compression ratio of the plate member is preferably 50 to 65%, and a soft material can be used as the plate member.