To provide a method for making a diaphragm wherein reduction and uniformization in the thickness of a polyimide thin-film used for the diaphragm are achieved and highly strong adhesion of the polyamide thin-film to a substrate is achieved to scarcely allow separation.

The preparation of the diaphragm includes a process for providing the substrate, a spin-coating process, a dehydration process, a masking process, and an etching process. In the spin coating process, a polyamic acid solution is supplied by the spin coating on the substrate. In the dehydration process, the polyamic acid solution is cyclodehydrated and polyimidated to form the polyimide thin-film. In the masking process, a mask is patterned on the surface opposing to the surface where the polyimide thin-film is formed in the substrate. In the etching process, the substrate is etched from the mask-forming side to the polyimide thin-film.