The present invention relates to an acrylic copolymer resin containing: 1) an alklyl (meth)acrylate-based monomer; 2) a (meth)acrylate-based monomer containing an aliphatic ring and/or an aromatic ring; and 3) at least an imide-based monomer or a styrene-based monomer, to a resin composition containing said acrylic copolymer resin and a resin containing an aromatic ring and/or an aliphatic ring in the main chain thereof, to an optical film comprising said resin composition, and to a liquid crystal display device comprising said optical film. The optical film according to the present invention has excellent heat resistance, optical transparency, etc.