Provided are a method and a device using same. The method, performed by a wireless power transmitter, for transferring wireless power to a wireless power receiver in a wireless power transmission system disclosed herein, comprises: entering a power transfer phase related to the transfer of the wireless power and transmitting auxiliary data transport (ADT) packets in the power transfer phase to the wireless power receiver, wherein the wireless power transmitter transmits the ADT packets to the wireless power receiver on the basis of frequency shift keying (FSK), and the maximum size of the ADT packets is determined on the basis of the number of cycles used in the FSK.