PURPOSE: To obtain a polyimide fiber suitable as a matrix resin for composite material, having excellent heat resistance, flame retardance and radiation resistance, by subjecting a thermoplastic polyimide to melt spinning under a specific condition.

CONSTITUTION: A thermoplastic polyimide comprising ≥85mol% repeating unit composed of a unit of the formula, having ≤7,000 poise melt viscosity at 100sec-1 at 400°C is subjected to melt spinning. Melt temperature (°C) ×melt retention time (minutes) is 2,000-4,200, pressure (kg/cm2) of resin to a filter before a spinneret is 80-200 and the resin is delivered at 5-12 m/minute delivery linear speed. The temperature of the spinneret is 390-420°C, the temperature of an atmosphere in a heating column is 250-400°C and thickening, gel formation and evolution of decomposed gas can be prevented.