To provide a plate member allowing any worker to form a curved face of a partitioning wall or a shuttering simply when forming the partitioning wall provided with the curved face or the shuttering without requiring labor of a skilled worker.

This plate member 1 is constituted by forming surface side slits 3 along the direction of long side or the direction of short side on a surface of a plate member main body 2 made of foaming resin at a proper interval and forming rear surface side slits 4 along the direction of long side or the direction of short side on a rear surface of the plate member main body at positions where they are arranged alternately with the surface side slits. These surface side slits 3 and rear surface side slits 4 have a V-shaped plane, respectively, and their tip parts are formed to be positioned on an inner side more than a central axis 6 of plate thickness and be curved. A reinforcing member 5 is detachably provided in the surface side slit 3 or the rear surface side slit 4.