PURPOSE: To make hardly cause buckling of foam resin filled diaphragm plate, by respectively filling in foamed resin of high density at the part near the root of cone or on the entire inner circumference in the cone and foamed resin of low density at the rest.

CONSTITUTION: Comparatively high density and high stress of foamed resin 4 is filled in the cone of 1' at the root including the part of buckling and comparatively low density and low stress foamed resin 5 is filled in the inside of other part 1". With the constitution like this, when the cone 1 is driven with very great acceleration via a bobbin 4 from a voice coil 3, a high stress produced by the bending of the part 1' near the root of the cone 1 can withstand.