PURPOSE: To achieve secure sealing of a fluid and detect the pressure of the fluid with high accuracy irrespective of variations in the pressure, by interposing a thin film between a fluid-detecting part and a casing, and sealing the detecting part to the casing by adhesion of the thin film to the casing.

CONSTITUTION: A pressure converter 1A comprises a thin film 11 interposed between a bottom wall of a casing 3 and a piezoelectric element 4. A back-up plate 10 is interposed between the thin film 11 and the bottom wall of the casing 3. The back-up plate 10 and the bottom wall of the casing 3, and also the back-up plate 10 and the thin film 11, are bonded to each other by soldering at peripheral edges thereof. The back-up plate 10 is provided with a through-hole 10a in the center thereof, for supplying a medium to the surface of the thin film 11 on the lower side of the piezoelectric element 4. The thin film 11 is provided with a clad construction composed of a copper film 11a on the lower side and a polyimide resin film 11b on the upper side, so as to be solderable and durable. Soldering by use of a solder SD enables secure liquid seal, which has been impossible with gaskets, and enables leakage of a refrigerant to be reduced to or below 10-6.