PURPOSE: To obtain a new hydroxyl group-containing lactam derivative capable of providing a highly branched polymer or thermosetting resin even when singly used.

CONSTITUTION: A compound of formula I [R is H or 1-20C alkyl; (n) is 2-13], e.g. 7-hydroxymethyl-2-oxohexamethyleneimine. The compound is obtained by oximating a hydroxyl group-containing cyclic ketones (e.g. 2- hydroxymethylcyclohexanone) of formula II and then subjecting the oximated product to ring-enlarging reaction by Beckmann rearrangement. A polyfunctional lactam derivative obtained by reacting the compound of formula II with a polyvalent carboxylic acid or polyvalent isocyanates is useful as a modifier for polyamide resin or curing agent, etc.