PURPOSE: To produce the subject filaments of high strength, which are used as a reinforcing material for rubber products such a tires in high yield by using a specific polyamide, cooling the filaments after spinning with an aqueous solution and taking up at a specific speed.

CONSTITUTION: A polyamide of more than 70 relative viscosity in formic acid (preferably from 80 to 150) is used to form filaments. The filaments are cooled by supplying an aqueous solution (which is composed of water, as a major component, finishing agents and solvents) through nozzles 15 and taken up with taking-up godet rolls 13a, 13b at a speed over 1,000m/min (preferably 1,500 to 3,000m/min) to give the subject filaments.