In nondestructive evaluation of concrete structures, the far-infrared region, including terahertz waves, which can penetrate concrete and measure the amount of corrosion in the internal steel, has attracted much attention. Magnetite has the potential to be used as a far-infrared detection device that meets the requirements for nondestructive evaluation devices, such as room temperature operation and portability, while also having a low environmental impact. In this study, the sensitivity of magnetite thin films with different concentrations of Pt to electromagnetic waves at a wavelength of 10.6 ㎛ was evaluated and compared: nanocomposite with Pt nanocrystals despersed in magnetite thin films were prepared by radio-frequency sputtering, electrodes were prepared by a photoresist process, and the resistance variation was recorded after irradiation with 10.6 ㎛ pulse electromagnetic waves. As a result, it was experimentally confirmed that the peak of response was the maximum at the amount of Pt added where the electrical resistivity reached 12,000 µΩcm, and the S/N ratio was the maximum at the amount of Pt added where the electrical resistivity reached 14,000 µΩcm. This indicates that Pt-doped magnetite with a Pt content of 14,000 µΩcm electrical resistivity is suitable as a far-infrared detector element material.