To provide an optical film of excellent heat resistance restraining discoloration at a high level, even when formed of a hydroxy acid-modified glucan derivative (for example, lactone-modified cellulose acylate).

This dope contains the hydroxy acid-modified glucan derivative constituted of a glucan derivative (cellulose acetate or the like), and a graft chain formed by graft-polymerizing a hydroxy acid component onto a hydroxyl group of the glucan derivative, and is formed flow-extendedly to make a ratio of the graft-polymerized hydroxy acid component get to 0.1-5 mol. in average in term of a hydroxy acid, per one molar glucose unit constituting the glucan derivative, in the hydroxy acid-modified glucan derivative. The optical film excellent in various characteristics such as the heat resistance and restraining the discoloration at the high level is obtained by using the dope.