Abstract: The resonance frequency of a resonant circuit section does not change even if the surface of an article whose theft is monitored is formed of any material. An antitheft tag mounted on the article (11) whose theft is monitored includes a resonant circuit section resonating with the radio wave having a particular frequency and transmitted from a transmitting antenna. An electromagnetic shield layer formed of an insulating material is interposed between the mounting surface of the article and the resonant circuit section. The electromagnetic shield layer may be formed of ferrite powder or soft magnetic powder which has a particle size of 10 &mgr;m or less and plastic or rubber or may be formed by laminating a first layer composed of a composite material and a second layer composed of plastic or rubber. Preferably, the soft magnetic powder is any of amorphous alloy, Permalloy, soft magnetic iron, silicon steel, Sendust alloy and Fe—Al alloy.

Abstract: A thermistor film is prepared by first preparing an alcohol solution is prepared by dissolving a metal compound in one or more kinds of polyvalent alcohols selected from the group consisting of ethylene glycol, diethylene glycol and glycerin. A coating solution is then prepared by adding and mixing an organic acid having a carboxyl group to the alcohol solution. The coating solution is coated on the surface of a heat-resistant substrate to form a coating film, the substrate on which the coating film is formed is dried and subjected to heat treatment to form a composite oxide precursor containing a metal of the above metal compound, and the precursor is calcined at a temperature of 600.degree. to 1000.degree. C. The method of the present invention can form a thermistor thin film which is dense and uniform over a wide range by simple and easy operations at a low cost.