An object is to achieve a radio power transmitting system capable of adaptively controlling power transmission efficiency, and the following means for achieving it is proposed. The power transmitting system includes a receiver having a loaded antenna and a transmitter whose transmitting frequency is variable, and it controls power transmission efficiency by using a modulated electromagnetic wave reflected from the receiver. A capacitive element and an inductive element are formed in the antenna, the transmitting frequency of the transmitter is set to be variable, the electromagnetic wave transmitted from the transmitter and reflected by the receiver is modulated, and the power transmission efficiency between the transmitter and the receiver is controlled by using the modulated reflection wave.