A wireless power transmitter includes a power conversion unit configured to transfer wireless power to a wireless power receiver by forming magnetic coupling with the wireless power receiver; and a communication/control unit configured to communicate with the wireless power receiver to control transmission of the wireless power and to perform transmission or reception of data, wherein the communication/control unit is further configured to receive, from the wireless power receiver, a received power packet (RPP) which informs a value of the wireless power received by the wireless power receiver, transmit a bit pattern to the wireless power receiver in response to the RPP, the bit pattern requesting attention from the wireless power receiver the wireless power transmitter, receive, from the wireless power receiver, a response packet to poll the reason of the attention, and transmit, to the wireless power receiver, a capability packet including information for a new target power level.