A reinforcing member for a flexible printed wiring board that maintains an electromagnetic wave shielding effect and a ground effect of the printed wiring board over a long period of time. A reinforcing member is disposed opposite a predetermined part of a ground wiring pattern and includes one surface opposing and in electrical conduction with the predetermined part of the ground wiring pattern. The other surface is in electrical conduction with an external ground member which is at a ground potential, the one surface and the other surface opposing each other. The reinforcing member includes a base made of conductive metal and a surface layer formed on a surface of the base to constitute at least a part of the other surface, the surface layer has higher conductivity and corrosion resistance than the base made of metal, and the surface layer is 0.004 to 0.2 μm thick.