PURPOSE: To easily detect the end of milling of an absorption thin film by a method in which a substrate has an opening part reaching a supporting thin film from its backside is formed in the vicinity of a part other than a transmission window opened in the substrate.

CONSTITUTION: For the first place, an SiN film 2, 2' is deposited on both sides of the (100) surface of a silicon substrate 1. Secondly, a resist 3 is coated on a backside thereof and a pattern for silicon etching is exposed and developed. This pattern has a transmission window and a part for forming an opening part. Then, by using the resist pattern as a mask, a film 2' is etched by a plasma-dry-etching method, and the resist 3 is removed. After that, an SiO2 film 4 is deposited on its backside by a hormal pressure CVD method or the like. By using a pattern by which the resist 3' is coated on the film 4 and a part for forming an opening part for milling end point measurement is exposed, exposure and development are conducted. Then, after etching and removing the film 4, the resist 3' is removed. After that, by using KOH and the like, the substrate 1 is etched. Under this state, a gold 5 is coated on the surface thereof and a resist 3" is coated.