To obtain a new organopolysiloxane-polylactone copolymer having hydrophilic group and suitably usable for a modifier of paper/pulp, a surface modifier of fiber, an additive for cosmetic/toiletry products, a modifier for rubber/resin, and a modifier for coating.

This new organopolysiloxane-polylactone copolymer is obtained by the steps of reacting an organopolysiloxane having at least one hydrogen atom in a molecule directly bonded to a silicon atom with a polylactone oligomer having at least one alkenyl group in a molecule, and introducing a substituent group having hydrophilicity into the reaction product.