PURPOSE: To stably form membrane by forming an etching prevention film and SiO2 film on silicone base upper surface, etching hole of pattern shape from the lower surface to the prevention film, and removing the part facing to the hole of the prevention film.

CONSTITUTION: On a silicon base 1 upper surface, Au film 2 is formed by sputtering and SiO2 film 3 is formed on it. Besides, SiN film 4 is formed on the lower surface of the base 1, and a resist pattern corresponding to a membrane shape is formed with photolithography, which is etched with CF4+O2 gas to form a pattern 4a. Next, the film 4 is etched with KOH solution with masking of the film 4. The membrane part 3a of the film 3 is then shielded from KOH solution. Next, film 2 of the membrane part 3a is removed by electrolysis. With this method the process is simplified and high quality membrane part 3a can be formed.