PURPOSE: To reduce the defect ratio of the title magnet in its injection molding operation and to increase the production efficiency by a method wherein, when a magnet composition is filled, it is molded by making the inside of a metal mold vacuum.

CONSTITUTION: When a magnet composition which is composed of a magnetic powder and a thermoplastic resin or a magnet composition which is composed of a magnetic powder, a thermoplastic resin and an additive is cast, in a molten state, into a metal mold 9 which has been evacuated to produce a vacuum, it is filled into a cavity in one go because the resistance of the air or a gas does not exist. At this time, the molten state of the magnet composition is uniform, and a temperature distribution is small. As a result, a molding defect duch as a weld-line silver streak, whitening, a retreat, a pore, a filling insufficiency, burning or like is not caused. In addition, since a gas generated from the thermoplastic resin, the additive or the like is sucked forcibly, an air vent used to remove the gas is not required, and the production of a burr can be prevented. In addition, since the molten state of the magnet composition is uniform and the temperature distribution is small, a molded product is cooled and solidified uniformly, and its performance can be enhanced.