To provide a curing compsn. showing little increase in tensile strength even exposed to heat after curing and suitable as a sealing material, and a curing agent for manufacturing a polymer contg. a thiol group.

This curing agent for a polymer contg. a thiol group is a compd. contg. not less than 2 isocyanate groups in a molecule and obtd. by reacting an aliphatic polyisocyanate, an alicyclic polyisocyanate, at least an arom. polyisocyanate shown by the formula wherein R1 and R2 are each a 1-3C alkyl group, and a compd. having not less than 2 active hydrogens in a molecule, and the concn. of unreacted polyisocyanate is not more than 1 wt.%. This curing compsn. comprises the curing agent and a polymer having not less than 2 thiol groups in a molecule.