PURPOSE: To provide a liquid isocyanate prepolymer that has a constant content of isocyanate group and can be stably stored, by reacting specified polyamide with the isocyanate mixture.

CONSTITUTION: Polyamine, which has two or more primary amino group connected to an aromatic group, is reacted with (A) di-phenylmethane di-isocyanate 50-100%, of which 3-5wt.% is formed of 2,2'-and-isomer, and (B) isocyanate mixture composed of polyphenyl polymethylene polyisocyanate at a residual quantity (in the case where this mixture includes di-phenylmethane di-isocyanate at 95-100%, 15% or more of di-isocyanate is composed of 2,2'- and 2,4'-isomer) so as to obtain the liquid isocyanate polymer, which contains isocyanate group at 10-30% and which can be stably stored.