A power feeding device utilizing an electromagnetic resonance coupling method and a contactless power feeding system can be provided. A coupling coefficient of electromagnetic induction coupling in the power feeding device and/or the inside of a power receiving device is optimized to improve electric power transmission efficiency of a resonance frequency regardless of positions of the power feeding device and the power receiving device. Provided is a power feeding device or a contactless power feeding system in which an S11 parameter which is a reflection component of electric power output from a high-frequency power source of the power feeding device is monitored, and one or both of positions of a transmission coil and a first resonant coil in the power feeding device and positions of a reception coil and a second resonant coil in a power receiving device are changed to adjust a coupling coefficient of electromagnetic induction coupling.