PURPOSE: To obtain the titled substance having high molecular weight, useful as an initiator for ω-lactam polymerization, by reacting a low-molecular weight acyllactam compound with a polyol or polyamine in the presence of a titanic acid ester as a catalyst.

CONSTITUTION: A compound shown by formula I [A is group shown by formula II (Y is 3W11C alkylene); n is ≥2 integer; Z is 0W20C hydrocarbon group] is reacted with a polyol or polyamine in the presence of a catalyst of a titanic acid ester to give the titled substance having high molecular weight. A polyether polyol containing one or more hydroxyl groups in one molecule or polyester polyol containing polyether segment is used as the polyol and a polyamine containing NH2 bonded to at least one H is used as the polyamine.