PURPOSE: To obtain an acoustic characteristic with low distortion and wide dynamic range, by using fine carbon hollow shpere as a filling agent for a diaphragm for a speaker and forming a diaphragm which is large in the internal loss, light in weight and strong.

CONSTITUTION: A fine carbon hollow sphere having 100μ of average paticle diameter and 0.20∼0.30 of particle specific gravity and polyvinylchloride (DOP/ wt%) are sufficiently kneaded with a heat roll at 150°C to obtain a sheet 200μ in thickness. The content of the fine carbon hollow sphere is 68wt%. This composite sheet is formed with a conventional high polymer forming method into a shape of speaker with a heat press at 180°C to obtain a diaphragm having 4.5×1010cm2/sec2 of specific modulus of elasticity and 0.065 of internal loss.