NOVELTY - A solar-cell back sheet has a weather-resistant resin layer (1), a water-vapor barrier layer (2) and an adhesive layer (3) sequentially laminated on a plastic-film base material (4). The weather-resistant resin layer is formed from a resin composition which comprises vinyl-type copolymer which is a graft polymer obtained by condensation and/or an addition reaction of copolymer (a) and polyester (b), polyisocyanate compound (B) having two or more isocyanate groups, and polyester (C) which has two or more functional groups capable of reacting with isocyanate group.    USE - Solar-cell back sheet is used for solar cell module (claimed).    ADVANTAGE - The solar-cell back sheet is economical, and is excellent in outdoor weather resistance for a long period of time.    DETAILED DESCRIPTION - A solar-cell back sheet has a weather-resistant resin layer (1), a water-vapor barrier layer (2) and an adhesive layer (3) sequentially laminated on a plastic-film base material (4). The weather-resistant resin layer is formed from a resin composition which comprises a vinyl-type copolymer which is a graft polymer obtained by condensation and/or an addition reaction of copolymer (a) and polyester (b), a polyisocyanate compound (B) having two or more isocyanate groups, and a polyester (C) which has two or more functional groups capable of reacting with isocyanate group. The number of isocyanate groups of component (B) is 0.1-5 times with respect to the total of the functional groups capable of reacting with an isocyanate group in components (A,C). The copolymer (a) is obtained by co-polymerizing unsaturated dibasic acid(s) chosen from maleic acid, maleic acid anhydride, fumaric acid, mesaconic acid and citraconic acid, and other monomers which have ethylenically unsaturated double bond and weight average molecular weight of 5000-500000. The polyester (b) has a functional group capable of reacting with a carboxyl group and/or an acid-anhydride group, and a functional group capable of reacting with an isocyanate group, and whose weight average molecular weight is 500-25000.    DESCRIPTION OF DRAWING(S) - The drawing shows a typical sectional view of the solar-cell back sheet.    Weather-resistant resin layer (1)    Water-vapor barrier layer (2)    Adhesive layer (3)    Plastic-film base material (4)