An acoustic diaphragm employed in a speaker or the like and a method for producing the acoustic diaphragm are disclosed. A diaphragm substrate is a diaphragm exhibiting air permeability such as a paper diaphragm. The substrate is processed with filling by microfibrilated pulp. For filling, the substrate is immersed in the liquid dispersion of the microfibrilated pulp and the pulp is deposited on the substrate under suction. When the microfibrilated pulp is deposited on the substrate under suction, any voids present in the substrate are closed by the microfibrilated pulp so that air tightness is improved without lowering the sound quality or detracting from desirable properties of the diaphragm, such as low density, high toughness and low losses.