To obtain a plasticizer capable of producing a cellulose ester film having reduced optical anisotropy and a small retardation value in the thickness direction by making the plasticizer include a specific compound.

This plasticizer comprises a compound of the formula (R1 to R4 are each H or a 2-18C acyl; R5 and R6 are each H, a 1-18C alkyl, a 2-18C alkenyl, a 6-18C aryl or a 7-18C aralkyl). A cellulose lower fatty acid ester and the compound of the formula are mixed with an organic solvent containing ≥50 wt.% of acetone or methyl acetate. The cellulose lower fatty acid ester is swollen, the swollen mixture is cooled to -100 to -10°C, the cooled mixture is heated to 0-200°C to prepare a cellulose ester solution in which the cellulose lower fatty acid ester is dissolved in the organic solvent. The solution is cast onto a substrate and the organic solvent is evaporated to preferably produce a cellulose ester film containing 2-25 wt.% of the compound of the formula, having ≤100 nm retardation value in the thickness direction.