A diaphragm has a through-hole for fixing a voice coil at its center part, and has a fixing part of an edge at its outer periphery which is substantially circular. The shape of the outer periphery of diaphragm is formed as follows: a first circle and a second circle overlap each other with their center points displaced in such a manner that at least one part of an outer periphery of the first circle and the second circle forms a part of the substantially circular outer periphery of the entire shape. Using this structure, signal reproduction characteristics can significantly be flattened by reducing the resonance level.