PURPOSE: To obtain the subject derivative which is a polyol resin applicable to a urethane resin, etc., having the biodegradability by previously carrying out the dehydrating and drying treatments of a reactional raw material consisting essentially of a cellulosic derivative and a lactone compound.

CONSTITUTION: This method for producing a lactone-modified cellulosic derivative is to previously carry out the dehydrating and drying treatments of a reactional raw material consisting essentially of a cellulosic derivative such as hydroxypropyl cellulose and a lactone compound such as -caprolactone, keep the moisture content thereof at ≤0.2%, react the resultant raw material at 130-180°C in a nitrogen atmosphere using tetrabutyl titanate or monobutyltin trioctylate, etc., as a catalyst while stirring the raw material and afford the derivative having <1% content of the homopolymer. Furthermore, the vacuum dehydration, azeotropic dehydration, a dehydrating agent such as phosphorus pentoxide, an adsorbent such as molecular sieve are used as dehydrating means.