An aliphatic polyisocyanate is prepared by adding water to a mixture containing an aromatic hydrocarbon or a nucleus substituted derivative thereof and hexamethylene diisocyanate, which is preliminarily heated at a temperature in the range from 100* to 140*C, under agitation in an amount of 3/1 to 2/1 in terms of molar ratio of hexamethylene diisocyanate to water to react with hexamethylene diisocyanate at said temperature.