A power receiving device includes a resonant coil, an electromagnetic coupling coil, a rectifier circuit, a smoothing circuit, a voltage converter circuit which converts a voltage value of DC power into another voltage value, a control circuit which obtains voltage and current values of the DC power input to the voltage converter circuit, calculates an impedance from the obtained voltage and current values, and generates a pulse width modulation signal for controlling an output voltage and an output current of the voltage converter circuit, and a load to which the output voltage and the output current of the voltage converter circuit are input. The duty ratio of the pulse width modulation signal is determined so that the impedance calculated by the control circuit becomes close to an impedance for the maximum efficiency in transferring the AC power. The power receiving device is included in a contactless power feeding system.