A glucan derivative (e.g., a cellulose acylate derivative) which is useful for a thermoplastic resin, is inhibited in hygroscopicity at a high level and is excellent in compatibility with a plasticizer or solubility in a solvent is provided. In the glucan derivative which comprises an acyl group and a graft chain formed from at least one graft component selected from a lactone and a hydroxycarboxylic acid, each bonding to a glucose unit of a glucan (e.g., a cellulose) constituting the glucan derivative, a hydroxyl group in the graft chain is protected with a protecting group. The graft chain may be formed from a C.sub.4-16lactone. The hydroxyl group in the graft chain may be protected with an acyl group (e.g., a C.sub.2-8alkylcarbonyl group such as acetyl group). The degree of substitution of the acyl group may be from 1 to 2.99, and the degree of substitution of the graft chain may be from 0.01 to 2.