PURPOSE: To make it possible to fix the corner parts of a plaster board quickly and accurately in case when the corner parts are formed in plaster boards, and fix the plaster boards with each other or join the plaster board with other members more firmly.

CONSTITUTION: A corner part is formed with a partition board 11 and a side board 12 which is located at least on one side of the partition board 1 where the surface opposed to the surface faced by the end of the side board 12 in the partition board 11 is assumed to be an indoor side wall 11A facing the indoor side. A fixing part 21 of the partition board which is in contact with the partition board 11 and fixed therewith and the side fixing part 22 fixed with the side board 12 provide a reinforcing member 20 whose horizontal cross section is L-shaped.