To provide a resin composition which has excellent heat resistance, dynamic characteristics, especially toughness, and characteristics as optical films exhibiting negative birefringence, to provide a film, to provide a high tough optical film comprising the same and exhibiting negative birefringence, and to provide a method for producing the same.

This highly tough film comprises 20 to 85 wt.% of a copolymer (a) having an α-olefin residue units : N-phenyl-substituted maleimide residue units = 49:51 to 35:65 (molar ratio) and having a weight-average mol.wt. of 5×103 to 5×106, and 80 to 15 wt.% of acrylonitrile-styrene copolymer (b) having an acrylonitrile residue units : styrene residue units = 36:64 to 50:50 (weight ratio) and a weight-average molecular weight of 5×103 to 5×106.