POWER TRANSMISSION SYSTEM

In order to provide an electric power transmission system capable of curbing a reduction in power transmission efficiency, an electric power transmission system of the present invention, which transmits electric energy via an electromagnetic field from a power transmission antenna (140) to a power receiving antenna (210), is characterized by including: an inverter unit (130) that converts DC voltage to AC voltage of a predetermined frequency to output; a power transmission-side control unit (150) that controls a drive frequency of the inverter unit (130) and a voltage value of DC voltage input to the inverter unit (130), and controls in such a way that a power value output from the inverter unit (130) is kept at a constant level; the power transmission antenna (140) to which AC voltage is input from the inverter unit (130); a rectifying unit (220) that rectifies an output of the power receiving antenna (210) to obtain DC voltage, and outputs the DC voltage; a step-up and step-down unit (230) that steps up or down DC voltage output from the rectifying unit (220) to output; a battery that is charged with an output of the step-up and step-down unit (230) ; and a power receiving-side control unit (250) that controls the step-up and step-down unit (230) in such a way as to charge the battery with a maximum power value.