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To produce magnetic field sensors for use on irregular surfaces, thin-film-type elements that can detect a magnetic field by the magneto-impedance (MI) effect were formed on 20-μm-thick polyimide substrates by sputtering. The thin-film MI element has a layered configuration with the conductive Cu film surrounded by ferromagnetic CoNbZr magnetic films. Excellent soft magnetic properties could be obtained for the CoNbZr amorphous magnetic films by optimizing preparatory conditions and inserting an SiO2 buffer layer between the element and the polyimide sheet. To detect the magnetic field acting on the MI element by means of an impedance change, the element was shaped into a meander pattern. The MI element exhibits an impedance change of more than 100% and shows good temperature stability compared to that of conventional thin-film MI elements fabricated on glass substrates.