PURPOSE: To ensure dimensional accuracy, to arrange the orientation of magnetic powder while simultaneously executing magnetization and to improve productivity by extruding a thermoplastic synthetic resin, into which magnetic powder is mixed, by an extruding die, passing the synthetic resin in a cooling sizing die, to which a magnetic pole is annexed, and cooling the synthetic resin while being fast stuck to the die by magnetic force.

CONSTITUTION: A water-cooling jacket 2 with a cooling-water inlet 2a and a cooling- water outlet 2b is fitted to the outer circumference of an extruding die D with a mandrel M. Annular permanent magnets 3aW3e, polarity thereof alternately changes, are mounted to the inner circumferential surface of the jacket 2, and the inner circumferential surface containing these permanent magnets is coated with a release type coating layer 5. Consequently, a magnetic-force sizing die 1 is constituted, a mixed resin magnetic composition P consisting of 700pts.wt. nonmagnetized ferrite powder, 100pts.wt. 6-nylon and 3pts.wt. plasticizer is fed in from one end section of the mandrel M and brought to a high-temperature fluidized state, and a tubular material P' cooled and cured is forwarded from the terminal of the die 1. Accordingly, the magnetic powder of the tubular material P' is oriented.