To obtain a cellulose ester derivative which has low hygroscopicity excellent dimensional stability and mechanical properties.

The derivative has a cross-linked alicyclic hydrocarbon group and an acyl group. For example, the cross-linked alicyclic hydrocarbon group may be adamantane represented by the formula (I) [wherein, R1, R2 and R3 are a hydrogen atom or a 1-20C hydrocarbon group; a bond may bind to the tertiary or secondary carbon atom] and a cross-linked alicyclic hydrocarbon group corresponding to its derivative or the like and the acyl group may be a 1-4C acyl group. The average substitution rate of the acyl group and the hydrocarbon group is 2/1-10/1, the average substitution of the acyl group is 1-2.9 and that of the hydrocarbon group is 0.1-1. The total of the average substitution may be about 2-3.