Mobile device, power supply platform, and power supply method for mobile device

To reduce power consumption when a mobile device is not set down, and to reliably detect when a mobile device has been set down so that problems can be prevented such as overheating when a conductive object such as a coil is set down. A mobile device 1 is set down on a power supply platform 2, power is transmitted from a power transmission coil 5 of the power supply platform 2 to a power receiving coil 4 of the mobile device 1, and the power is supplied to the mobile device 1. The power supply platform 2 temporarily supplies alternating current power to the power transmission coil 5 for a predetermined period, detection signals such as the current of the power transmission coil 5 are detected while alternating current power is being supplied to the power transmission coil 5, ID signals transmitted from a mobile device 1 are detected when a set down mobile device 1 has been detected based on the level of the detection signals, alternating current power is supplied from the power transmission coil 5 to the power receiving coil 4, and the mobile device 1 is powered up.