NOVELTY - A solar-cell back surface protection sheet comprises weather-resistant flame-resistant resin layer (1), plastic film (2), and easily-adhesive layer (3). The layer (1) contains phosphorus-based flame retardant (A) chosen from phosphazene compound, phosphinic acid compound, and melamine (poly)phosphate, and acrylic resin (B). The acrylic resin (B) has glass transition temperature of 0-70 degrees C, weight average molecular weight of 15000-150000, and hydroxyl value of 2-30 mg potassium hydroxide/g. The total concentration of phosphorus obtained from flame retardant (A) is 2.1-14.2 wt.%.    USE - Solar-cell back surface protection sheet is used for solar-cell module (claimed).    ADVANTAGE - The economical solar-cell back surface protective sheet has favorable flame-retardant properties based on UL-1703 spread of flame test, and excellent resistance with respect to moist heat and outdoor weather.    DETAILED DESCRIPTION - A solar-cell back surface protection sheet comprises weather-resistant flame-resistant resin layer having film thickness (t mu m), plastic film, and easily-adhesive layer. The weather-resistant flame-resistant resin layer comprises surface (A) of one side of the solar cell back surface protection sheet, and the easily-adhesive layer comprises surface (B) of the solar cell back surface protection sheet. The weather-resistant flame-resistant resin layer contains phosphorus-based flame retardant (A) chosen from phosphazene compound, phosphinic acid compound, and melamine (poly)phosphate, and acrylic resin (B). The acrylic resin (B) has glass transition temperature of 0-70 degrees C, weight average molecular weight of 15000-150000, and hydroxyl value of 2-30 mg potassium hydroxide/g. The weather-resistant flame-resistant resin layer has film thickness (t) of 2.5-20% of total film thickness of solar cell back surface protection sheet. The total concentration of phosphorus obtained from phosphorus-based flame retardant (A) in the weather-resistant flame-resistant resin layer is 2.1-14.2 wt.%. An INDEPENDENT CLAIM is included for solar-cell module, which has solar-cell surface-sealing sheet located at light-receiving surface of solar cell, sealing-material layer located at light-receiving surface of the solar cell, solar-cell element, sealing-agent layer located at non-light-receiving surface of the solar cell, and solar-cell back surface protection sheet which is in contact with non-light-receiving surface of sealing agent layer. The layer (1) of solar-cell back surface protection sheet is located far away from solar cell surface sealing sheet.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the solar-cell back surface protective sheet.    Weather-resistant flame-resistant resin layer (1)    Plastic film (2)    Easily-adhesive layer (3)