PURPOSE: To obtain a nylon-4,6 uniformly filled with a reinforcing material which has an optimal length when it is a glass or carbon fiber and being excellent in mechanical properties and less colored.

CONSTITUTION: The production process comprises polycondensing a salt of 1,4-diaminobutane with adipic acid in the presence of excess 1,4-diaminobutane and water at polyamide-forming temperature under superatmospheric pressure to give a precondensate having a relative viscosity of ≤1.60, polycondensing the precondensate in a solid phase in the presence of steam at 220-270°C to obtain a nylon-4,6 having a relative viscosity of 2-26, incorporating a reinforcing material (e.g. glass fiber, wollastonite or kaolin) in the melt into the nylon-4,6 and polycondensing the filled nylon-4,6 in solid phase in the presence of steam at 220-270°C to a relative viscosity of at least 3.2.