PURPOSE: To enable assemble with high accuracy, by positioning a gap between a voice coil and a field in the operating state of a loud speaker.

CONSTITUTION: A loud speaker 21 is driven at a low resonance frequency, a diaphragm 8 of the loud speaker bein on the way of assembling at a counter position is vibrated back and forth, and the voice coil 9 is moved in the magnetic gap 6 of the field 1. With the coil 9 inserted in the gap 6, the fields is moved minutely toward, e.g., right direction and when an abnormal sound is generated, the operation of a shift mechanism 26 is interrupted and the position is stored at a control circuit 24. Then, the field 1 is minutely moved left until the abnormal sound is generated, to stop the mechanism 26, at the point of time, the field is returned left to an intermediate position between the position and that stored at the minute shift toward right direction. The similar operation is applied also in the Y axis to complete the ideal positioning of the voice coil 9.