PURPOSE: To rapidly and readily obtain the title nylon 66 having high polymeriza tion degree and useful as tire cords, etc., by using a strong base as a hydrogen halide trapping agent and polymerizing an adipoyl halide with hexamethy lenediamine.

CONSTITUTION: (A) An adipoyl halide is polymerized with (B) hexamethy lendiamine in a solution containing (C) a solvent without any active proton. In the process, (D) a strong base is used as a hydrogen halide trapping agent to afford the aimed nylon 66. Further, the component (C) is preferably a tertiary amine having ≥9.83 acid dissociation constant pKa without any active proton. Furthermore, the component (B) is preferably mixed with the component (A) below a temperature at which dehydrohalogenation reaction is not caused from the component (A) and then heated.