A diaphragm for a speaker is made by forming acrylic foam. As the material of the acrylic foam, using maleic anhydride and methacrylic amide are contained. Thereby, the coefficient of saturated water absorption can be reduced. When the diaphragm thus manufactured is used for a speaker device, the decrease of strength due to the water absorption and the deterioration of the sound quality can be prevented even in a humid environment.