To make a diaphragm thin and light in weight, to make it into complicated form, to make it flexible, to improve vibration efficiency, to increase the sound pressure and to expand a frequency range to provide a prescribed sound pressure by making the diaphragm of resin and executing magnetic processing.

A magnet 23 is arranged at the outer periphery of a coil 15 while keeping a loop-shaped gap 21, a stepped part 25 is formed at the inner periphery of the lower case 5, and a diaphragm 27 is arranged at this stepped part 25. A plane part 28 and an additional mass part 29 consisting of this diaphragm 27 are constituted by integral molding, engineering plastics such as 'Noryl(R)' (PPO), nylon and liquid crystal polymer(LCP) are used as its materials, and magnetic materials are plated on the outer surfaces of plane part 28 and additional mass part 29. This application of magnetic materials is not only metal plating but also can be replaced with a method such as sticking foil-shaped materials or applying magnetic paste. Besides, as the magnetic materials, there is Ni-Fe alloy, for example, and it is preferable its ratio is 75% to 25%.