To prevent the occurrence of a crack or an edge joint lack of a pau lownia bottom plate used in a furniture and the like due to an elongation or a contraction of the plate according to a temperature at the time of elapsing number of days even if a dried plate is used.

Lye is removed from a paulownia plate material 1 cut in a thickness of about 10 mm, the material is then dried, cut in a length of 40 to 90 cm, hot pressed for a predetermined time by a hot press 4. Then, its edge joints are removed by both side cutting edges in a length width direction by a lip saw 5, adhered by an adhesive, aligned in parallel, press adhered by a high-frequency welder 6 and manufactured. In this case, the treatment by the press 4 and the treatment by the welder 6 may be executed in a reverse order.