To provide a new rosin-based resin having excellent compatibility with a reactive diluent such as a multifunctional (meth)acrylate compound such as dimethylolpropane tetraacrylate or pentaerythritol tetraacrylate, and improving adhesiveness of an active energy ray-cured film to a polyolefin substrate.

A rosin-based copolymer (A) is used, which is obtained by carrying out a copolymerizing reaction of an acryloyl or methacryloyl-containing rosin derivative (a-1) prepared by subjecting rosins and an acryloyl or methacryloyl-containing monovinyl ether compound to a hemiacetal reaction, with a monofunctional vinyl compound (a-2).

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