In the rolling process, a rolling amount at each time of rolling by a rolling machine is set to 1 μm to 20 μm, and a magnesium substrate is heated by a constant temperature oven and rolled by rollers. Thus, a magnesium sheet of 30 μm to 100 μm is produced. This magnesium sheet is used to form a bobbin and a diaphragm integrally molded with each other, and the diaphragm is molded into a semi-dome shape or a dome shape. Thus, a bobbin integrated type magnesium diaphragm having the semi-dome shaped diaphragm or the dome shaped diagram is manufactured. The bobbin integrated type magnesium diaphragm is applied to a dynamic speaker device. As a result, the high-quality dynamic speaker device, which realizes high rigidity, high sensibility, high internal loss, less distortion and the like, is obtained.