PURPOSE: To achieve high efficiency in manufacturing of a long rod-shaped body having the small diameter by using N-acetyl--caprolactam as a promoter when lactam is injected into a mold together with anion polymer catalyst and the promoter, performing polymerization, and the rod-shaped molded body having the small diameter comprising polyamide is formed.

CONSTITUTION: In a beaker made of stainless steel, 100 pts.wt. of -caprolactam is put, and the temperature is adjusted to 130°C. Sodium hydride of 2 pts.wt. and N-acetyl--caprolactam of 0.1 pts.wt. are added and mixed. The mixture is injected into a metal mold, whose temperature is adjusted to 150°C. The metal mold is used for round-holding molding and has the diameter of 30mm and the length of 1,500mm. The molded body, which is taken out of the metal mold after 5 hours, has the clean cylindrical shape without recess parts around the outer surface, and there is no voids in the inside.