To provide a technique which can effectively prevent deformation of a light reflector plate made of plastic and machined into a three-dimensional shape and also prevent this reflector plate from making fragile points and bending at these positions deforming the whole reflector plate.

A bending area 6 is formed on a plastic light reflecting film or a sheet 2 by forming folding lines consisting of cut sections or recesses continuously or intermittently made along a straight line, and making some of them 6a, 6b close to each other in parallel. Then, the film or sheet is bent along this bending area.