A method for transmitting wireless power by a wireless power transmitter , the wireless power transmitter including a power transmitter and a communication unit , the method comprising: detecting a change in impedance of the wireless power transmitter caused by a placement of a wireless power receiver by periodically applying detection power to the power transmitter (operation S301; S607; S701, S703; S1007); applying driving power for driving the wireless power receiver, to the power transmitter, in response to detecting the change; receiving a wireless power transmitter search signal from the wireless power receiver , wherein the driving power is used to transmit the wireless power transmitter search signal from the wireless power receiver to the wireless power transmitter(operation S1023), wherein the driving power is used by the wireless power receiver for transmitting the wireless power transmitter search signal to the wireless power transmitter; transmitting a wireless power transmitter search response signal corresponding to the wireless power transmitter search signal; establishing a communication connection with the wireless power receiver based on the wireless power transmitter search response signal and the wireless power transmitter search signal; registering the wireless power receiver in a wireless power network managed by the wireless power transmitter by transmitting and receiving signals using the communication connection; and applying charging power for charging the battery of the wireless power receiver, to the power transmitter (operation S309; S638, S642; S1041, S1045), wherein a power level of the detection power is smaller than a power level of the driving power, wherein a power level of the charging power is greater than the power level of the driving power. detecting a change in impedance of the wireless power transmitter caused by a placement of a wireless power receiver by periodically applying detection power to the power transmitter (operation S301; S607; S701, S703; S1007); applying driving power for driving the wireless power receiver, to the power transmitter, in response to detecting the change; receiving a wireless power transmitter search signal from the wireless power receiver , wherein the driving power is used to transmit the wireless power transmitter search signal from the wireless power receiver to the wireless power transmitter(operation S1023), wherein the driving power is used by the wireless power receiver for transmitting the wireless power transmitter search signal to the wireless power transmitter; transmitting a wireless power transmitter search response signal corresponding to the wireless power transmitter search signal; establishing a communication connection with the wireless power receiver based on the wireless power transmitter search response signal and the wireless power transmitter search signal; registering the wireless power receiver in a wireless power network managed by the wireless power transmitter by transmitting and receiving signals using the communication connection; and applying charging power for charging the battery of the wireless power receiver, to the power transmitter (operation S309; S638, S642; S1041, S1045), wherein a power level of the detection power is smaller than a power level of the driving power, wherein a power level of the charging power is greater than the power level of the driving power. <EOS>