POWER SUPPLY DEVICE, POWER SUPPLY SYSTEM, AND ELECTRONIC DEVICE

There are provided an electric power feed apparatus, an electric power feed system, and an electronic apparatus which are capable of performing transmission efficiency control corresponding to positions of apparatuses when electric power transmission is performed between the apparatuses through a magnetic field. The electric power feed apparatus includes an electric power transmission section including an electric power transmission coil for performing electric power transmission through a magnetic field and an auxiliary resonance section including one or a plurality of resonators. A main resonant frequency in a main resonance operation with use of the electric power transmission coil during the electric power transmission and an auxiliary resonant frequency in the resonator are different from each other. A relationship (corresponding characteristics) between a relative position between the electric power feed apparatus (an electric power transmission side) and the electronic apparatus (an electric power reception side) and transmission efficiency during electric power transmission is varied by adjustment of such a difference between the resonant frequencies.