A cellulose derivative which has a suitable elastic modulus and viscosity when it is dissolved in water and is useful as an adhesion barrier. The cellulose derivative has a molecular weight of 5 x 10 3 to 5 x 10 6 and is composed of a repeating unit represented by the following formula (1): wherein R 1 , R 2 and R 3 are each independently selected from the group consisting of the following formulas (a), (b), (c) and (d) in predetermined amount: €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ-H€ƒ€ƒ€ƒ€ƒ€ƒ(a) €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ-CH 2 -COOH€ƒ€ƒ€ƒ€ƒ€ƒ(b) €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ-CH 2 -COOX€ƒ€ƒ€ƒ€ƒ€ƒ(c) (X in the formula (c) is an alkali metal, and R 4 and R 5 in the formula (d) are each independently an alkyl group or alkenyl group having 10 to 28 carbon atoms).
