To obtain the subject compound useful as a curing agent for a highly functional OH-containing compound by blocking a diisocyanate and/or a polyisocyanato isocyanurate with a malonic ester.

A polyisocyanate mixture comprising (A) a diisocyanate of the formula: OCN-R-NCO (wherein R is a 6-14C (cyclo)aliphatic hydrocarbon group) and/or (B) a polyisocyanato isocyanurate of formula I (wherein n is 1-3) is perfectly or partially blocked with (C) a blocking agent comprising a malonic ester of formula II (wherein R1 is a 1-10C alkyl or formula III) to such a degree that the rate of the free NCO groups of the reaction product is at most 4%. Component A is desirably isophorone diisocyanate or the like. Component C is desirably di-t-butyl malonate or the like. When a mixture of components A and B is used, it is suitable that component A is partially trimerized in situ in the presence of a trimerization catalyst until the desired content of component B is reached.