PURPOSE: To obtain the subject fiber having smooth surface and excellent mechanical properties, etc., by forming a core component with a thermoplastic polymer containing a specific amount of magnetic powder and forming a sheath component with a thermoplastic polymer having a softening point or melting point lower than those of the thermoplastic polymer of the core component.

CONSTITUTION: The objective fiber is produced by forming a core component with a thermoplastic polymer (e.g. polyolefin polymer) containing 5-70wt.% of magnetic powder (preferably samarium-cobalt magnetic material) and forming a sheath component with a thermoplastic polymer (preferably a polymer similar to the core component polymer) having a softening point or melting point lower than those of the thermoplastic polymer of the core component by ≥10°C. The ratio of (core component):(sheath component) is preferably (3:7) to (6:4). A magnetic fiber aggregate having excellent shape-retainability can be produced by heating a fiber aggregate containing ≥50wt.% of the above fiber at the welding temperature of the sheath component of the fiber to effect the integration of the fibers.