PURPOSE: An epoxy resin curing agent capable of providing cured products excellent in flexibility, peeling strength, etc., consisting mainly of a polybutadiene having the skeleton modified with a compound having at least one thiol group and at least one amino group in the molecule.

CONSTITUTION: 100pts.wt. polybutadiene is reacted with about 5W50pts.wt. compound having at least one thiol group and at least one amino group in the molecule (e.g., 2-aminoethanethiol) in the presence of a peroxide catalyst (e.g., benzoyl peroxide) or a radical initiator (e.g., p-toluenesulfonic acid) or/and a radical stabilizer (e.g., triallylamine) at a temperature of about 40W150°C in a nitrogen stream. Then, the purpose epoxy resin curing agent is prepared by mixing the above skeleton-modified polybutadiene as a principal component with, if necessary, a compound having at least one amino group (e.g., ethylenediamine), or like compounds.