To provide a resin composition which exhibits negative birefringence and is excellent in thermal stability as well as in transparency, heat resistance and mechanical strength, and an optical film composed of the resin composition.

The transparent resin composition comprises 100 pts. wt. of the total of 30-95 wt% of an N-phenyl-substituted maleimide-olefin copolymer having a number-average molecular weight of at least 1×103 and at most 5×106 and 70-5 wt% of a styrene copolymer of a specific structure and, incorporated therewith, 0.001-2 pts. wt. of a phenolic antioxidant and/or a phosphorus antioxidant, and exhibits negative birefringence. The optical film is composed of the resin composition.