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A novel foreign-metal detection-system for wireless power transfer (WPT) is described. Foreign metal was heated only when placed between the primary coil and the secondary coil. By having the metal in that position, the quality factor of the secondary coil (Q2) decreased rather than other parameters, so Q2 was a suitable parameter for detecting the metal. We developed s Q2 measurement circuit that works without any battery in the secondary device, and also developed a prototype WPT system with the Q2 measurement circuit. Using this system, foreign metal that might be heated could be detected before heating.