PROBLEM TO BE SOLVED: To improve the protective effect of an induction power supply system.SOLUTION: A method used in an induction power supply system, and detecting whether or not an invading metal exists in the transmission region of the induction power supply system, includes a step of stopping the driving of the power supply side coil of the induction power supply system, by interrupting at least one drive signal thereof, a step of acquiring a first attenuation gradient in a first period when the driving of the power supply side coil is stopped, and acquiring a second attenuation gradient in a second period when the driving of the power supply side coil is stopped, and a step of determining whether or not an invading metal exists in the transmission region of the induction power supply system, according to the first and second attenuation gradients.SELECTED DRAWING: Figure 5