Disclosed is a highly-adhesive agent for a solar-cell back-side protective sheet. Said highly-adhesive agent overcomes hitherto existing problems and exhibits excellent adhesiveness and adhesion durability. Also disclosed are a solar-cell back-side protective sheet and a solar cell module using said solar-cell back-side protective sheet. The disclosed highly-adhesive agent exhibits a specific glass transition temperature, number-average molecular weight, and hydroxyl number, and contains, in a specific ratio: a (meth)acrylic copolymer (A) that contains no (meth)acryloyl groups; a compound (B) that does contain (meth)acryloyl groups; and a polyisocyanate compound (C).