NOVELTY - A back sheet consists of a weather-resistant resin layer (1), a water-vapor barrier layer (2), adhesive layer (3), and plastic film base material (4). The resin layer contains a resin composition comprising a vinyl-type copolymer, a polyisocyanate compound, and polyester having two or more functional groups reactive with isocyanate group. The number of isocyanate groups in the polyisocyanate compound with respect to the total number of functional groups of the vinyl-type copolymer and polyester reactive with isocyanate group is 0.1-5.    USE - Back sheet is used as back-surface protection sheet of solar cells used in solar-cell modules (claimed).    ADVANTAGE - The back sheet has excellent economical efficiency and long-term outdoor weather resistance.    DETAILED DESCRIPTION - A back sheet consists of a weather-resistant resin layer, a water-vapor barrier layer, adhesive layer, and plastic film base material. The resin layer contains a resin composition comprising a vinyl-type copolymer, a polyisocyanate compound having two or more isocyanate groups, and polyester having two or more functional groups reactive with isocyanate group. The vinyl-type copolymer is a graft polymer obtained by condensation and/or addition reaction of copolymer (a) having a weight average molecular weight of 5000-500000, and polyester (b) having a weight average molecular weight of 500-25000. Copolymer (a) is obtained by copolymerizing unsaturated dibasic acid and other monomers having ethylenically unsaturated double bond. The unsaturated dibasic acid is maleic acid, maleic acid anhydride, fumaric acid, mesaconic acid, citraconic acid, and/or citraconic acid anhydride. Polyester (b) has a functional group reactive with a carboxyl group and/or an acid anhydride group, and a functional group reactive with isocyanate group. The number of isocyanate groups in the polyisocyanate compound with respect to the total number of functional groups of the vinyl-type copolymer and polyester reactive with isocyanate group is 0.1-5. An INDEPENDENT CLAIM is included for solar-cell module.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the back sheet.    Weather-resistant resin layer (1)    Water-vapor barrier layer (2)    Adhesive layer (3)    Plastic film base material (4)