A wireless power transmitter and a wireless power receiver are discussed. The wireless power transmitter can include a power converter configured to transfer wireless power to a wireless power receiver, and a communicator/controller configured to control the wireless power. The wireless power transmitter can receive a received power (RP) packet which informs a received power value, transmit in response to the RP packet a bit pattern related to requesting permission to communicate, receive a response packet which invites the wireless power transmitter to send a data packet, transmit a specific packet includes a specific power level in response to the response packet, and negotiate a guaranteed power level with the wireless power receiver. The guaranteed power level can be less than or equal to the specific power level.