A power supply device has a primary coil, a detection coil and a controller. The detection coil detects and outputs as a change in induction voltage any load fluctuation resulting from change in distance between the primary coil and a secondary coil. The controller controls, based on the change in the induction voltage output by the detection coil, an output of an oscillation circuit to hold power supplied to the secondary coil to a constant level. Therefore, even if the distance between the primary coil and the secondary coil changes, power supplied to battery connected to the secondary coil is stable. In particular, when the distance between the primary coil and the secondary coil becomes far, the controller increases the output of the oscillation circuit.