PURPOSE: To make an enzymatic reaction in an organic solvent possible and to prepare a carboxylic acid ester by a modification lipase, by reacting a carboxylic acid with an alcohol in the presence of a specific modification lipase in an organic solvent to esterify it.

CONSTITUTION: In the presence of a modification lipase obtained by reacting a lipase with an active derivative of a polyalkylene glycol containing a hydrophobic group at the end, a carboxylic acid is reacted with an alcohol in an organic solvent, it is esterified, or a carboxylic acid ester is reacted with another carboxylic acid or alcohol, so that it is subjected to ester exchange to give an ester. Since the modification lipase is modified with a straight chain comprising a substituted polyalkylene glycol having both properties of hydrophobic nature and hydrophilic nature, it is soluble in both water and an organic solvent, a high-order structure of lipase of its own is protected with the straight chain, and prevents deactivation caused by contact with the organic solvent.