To provide an optical film excellent in heat-resistant stability and humidity stability of optical characteristics.

In this optical film formed of a hydroxy acid-modified glucan derivative comprised of a glucan derivative (cellulose acetate and the like) and a graft chain, which is formed by graft polymerization of hydroxy acid constituents to hydroxy groups of the glucan derivative, a ratio of the graft-polymerized hydroxy acid constituents in the modified glucan derivative is regulated (e.g. the ratio is regulated to about 0.1-5 mol on the average in terms of hydroxy acid amount per 1 mol of glucose units constituting the glucan derivative).