To prevent generation of deformation while increasing rigidity and distributing the external force in directions except for the vertical direction by providing a frame member for covering an opening part peripheral edge with a surface structure based on a triangle shape inside thereof, and forming the frame member out of the resin molding.

A frame member 10 for covering an opening part peripheral edge of a building is formed with the surface structure by assembling plural reinforcing piece plates 40 between a surface finish plate 20 and a fitting plate 30 so that a cross section in a direction at a right angle against the longitudinal direction is basically formed into a triangle shape, and formed of a resin molding. A fixing fitting is previously fitted to a building frame body at the opening part thereof, and the frame member 10 is inserted into the opening part, and a projecting piece of the fixing fitting is pinched between the surface finish plate 20 and the fitting plate 30, and fixed to the building frame body through the fixing fitting. Since the frame member 10 is provided with the triangle surface structure of the reinforcing plates 40 inside thereof, the frame member 10 has a large rigidity, and the external force in directions except for the vertical direction is distributed in plural directions so as to prevent the easy generation of deformation. The device has also excellent evenness, water resistance, appearance, and the fitting work thereof is facilitated.