PURPOSE: To accurately insert a voice coil into a magnetic clearance without requiring any high dimensional accuracy to the parts, by positioning the voice coil to the center of the magnetic clearance through a uniform negative pressure in the dome of a dome-shaped diaphragm and the self-centering function of a magnetic fluid.

CONSTITUTION: A magnet 5 is put between a lower plate 2 having a center pole 1 and a ring-shaped upper plate 3, and a magnetic clearance 4 is formed with the outer wall of the pole 1 and the inner wall of the plate 3 opposing to each other. A hole 12 is installed through the pole 1, and a magnetic fluid 13 is poured into and maintained in the clearance 4, and then, a diaphragm 7 is placed on the plate 3 so that its voice coil 11 may be inserted slightly into the clearance 4. Then, the coil 11 is positioned to a location in the clearance 4 by mean of vacuum sucking with an adequate pressure through the hole 12, and then, the diaphragm 7 is supported by the plate 3. Thereafter, the coil 11 is inserted into the clearance 4 with the negative pressure in a dome part and the centering function of the magnetic fluid 13 without requiring any high accuracy to the parts.