To reinforce firmly the inner peripheral edge of an opening provided in a panel and to improve the operability of reinforcement by sticking a corner member to the rear side of each corner angle part of the opening and by fitting a side member to each side of the opening from inside.

A corner member 1 formed of iron or resin is stuck with an adhesive to the rear side of each corner angle part of an opening H provided in a panel P. Besides, a side member 2 having the length of one side of the opening H and having a section so shaped that it can be fitted in the inner periphery of one side is formed beforehand out of aluminum or hard resin by extrusion molding. By this constitution, construction can be executed without using screws or the like, operability is improved thereby and the inner peripheral edge of the opening H can be reinforced firmly. It is more preferable that a projecting part 1d is formed in the corner member 1 and the end part of the side member 2 is made to fit to the projecting part 1d.