A wireless power receiver to wirelessly receive power from a wireless power transmitter which receives power from a power supply device and transmit the power to a load, the wireless power receiver comprising: a receiving unit including a reception resonant coil configured to receive AC power from the wireless power transmitter through resonance between the reception resonant coil and a transmission resonant coil of the wireless power transmitter which are resonance-coupled with each other and configured to transmit power by operating at a resonant frequency; a rectifying unit to rectify the received AC power to DC power; characterized by a power managing unit (360, 370) to manage the power transmitted to the load based on the rectified DC power, wherein the power managing unit (360, 370) comprises: a controller (362, 371) to detect the rectified DC power and generate a signal to adjust the AC power to be received from the wireless power transmitter according to the rectified DC power; a modulation unit to transmit the signal to the wireless power transmitter through out-of band communication at a specific frequency band other than the resonant frequency band; and a voltage restriction unit to absorb an over-voltage equal or greater than a second threshold value, if the rectified DC power is the second threshold value or more, wherein the power managing unit (360, 370) is configured to transmit a signal to increase power to the power supply device and/or the transmitter to maintain the power transmitted to the load to a first threshold value, if the rectified DC power is less than the first threshold value, and the first threshold value is a minimum power required to normally operate the load, and wherein the power managing unit (360, 370) is configured to transmit a signal to decrease power to the power supply device and/or the transmitter to maintain the power transmitted to the load to the first threshold value, if the rectified DC power is equal to or greater than the second threshold value, and the second threshold value is greater than the first threshold value. a receiving unit including a reception resonant coil configured to receive AC power from the wireless power transmitter through resonance between the reception resonant coil and a transmission resonant coil of the wireless power transmitter which are resonance-coupled with each other and configured to transmit power by operating at a resonant frequency; a rectifying unit to rectify the received AC power to DC power; characterized by a power managing unit (360, 370) to manage the power transmitted to the load based on the rectified DC power, a controller (362, 371) to detect the rectified DC power and generate a signal to adjust the AC power to be received from the wireless power transmitter according to the rectified DC power; a modulation unit to transmit the signal to the wireless power transmitter through out-of band communication at a specific frequency band other than the resonant frequency band; and a voltage restriction unit to absorb an over-voltage equal or greater than a second threshold value, if the rectified DC power is the second threshold value or more, wherein the power managing unit (360, 370) is configured to transmit a signal to increase power to the power supply device and/or the transmitter to maintain the power transmitted to the load to a first threshold value, if the rectified DC power is less than the first threshold value, and the first threshold value is a minimum power required to normally operate the load, and wherein the power managing unit (360, 370) is configured to transmit a signal to decrease power to the power supply device and/or the transmitter to maintain the power transmitted to the load to the first threshold value, if the rectified DC power is equal to or greater than the second threshold value, and the second threshold value is greater than the first threshold value.