According to the first character of the invention, a composition for a backing film is provided which comprising a pigment (p) and a synthetic resin binder (B), the pigment (p) contains a lead-free rust preventing pigment for producing a corrosion protecting mirror, the corrosion protecting mirror is producing by forming a metal film on a glass substrate and a lead-free backing film on the metal film, pigment (p) is dispersed into the synthetic resin binder (B) in a weight ratio of 1.2 to 4.0, and the lead-free rust preventing pigment contains a molybdenum compound. In addition, a corrosion protecting mirror cures after coating the backing film composition to form a backing film is provided accord to the first character of the invention. |