The contactless power feeding system includes a power transmitting device including an AC power source, a power transmitting element transmitting an AC power and a first microprocessor generating a transmission signal, and a power receiving device including a power receiving element receiving the AC power, a rectifier circuit, a smoothing circuit, a voltage conversion circuit, a second microprocessor generating a response signal in accordance with the transmission signal, a charge control circuit changing a charging rate for a power storage device in accordance with the response signal and the power storage device whose charging is controlled by the charge control circuit. Then, a resistance value of the power storage device changes, an impedance changes, and a modulation signal is generated. The generated modulation signal is transmitted from the power receiving device to the power transmitting device and is processed by the first microprocessor.