A magnet assembling device includes a rounding die into which a rectangular and flexible rubber magnet can be mounted in an annular manner; a die guide for guiding the rubber magnet mounted in the rounding die into a motor case; a push punch including a punching portion for inserting the rubber magnet mounted in the rounding die into the die guide and for pushing the rubber magnet out of the die guide into the motor case; and a magnetizing head for magnetizing the inner peripheral surface of the rubber magnet. The die guide is formed in a tapered shape such that the inside diameter dimension of the side thereof from which the rubber magnet is inserted is larger than the inside diameter dimension of the side thereof from which the rubber magnet is discharged. Further, the magnetizing head is mounted on the push punch in such a manner that the magnetizing portion of the magnetizing head projects toward the rubber magnet beyond the punching portion of the push punch, and the magnetizing portion of the magnetizing head can be brought into close contact with the inner peripheral surface of the rubber magnet in the minimum diameter portion of the die guide.