Power feed device, power receiving device, and wireless power feed system

The invention relates to a power feed device, a power receiving device, and a wireless power feed system. The power feed device includes: a power generator configured to generate power that should be fed; a power feed element configured to be formed of a coil fed with power generated by the power generator; a resonance element configured to be coupled to the power feed element by electromagnetic induction; and a variable matching unit configured to include a function for impedance matching at a point of feed of the power to the power feed element, wherein a diameter of the power feed element is changeable, and the variable matching unit is capable of changing the diameter of the power feed element.