Provided in the present disclosure are a method by which a wireless power transmitter transmits wireless power in a wireless power transmission system, and a device using same, the method comprising: establishing a power transmission contract with a wireless power receiver in a negotiation phase; and transmitting, to the wireless power receiver, wireless power on the basis of the power transmission contract in a power transmission phase, wherein: the wireless power transmitter receives request information from the wireless power receiver in the negotiation phase; the request information is information related to the unit size of at least one piece of auxiliary data transport (ADT) information transmitted by means of the wireless power transmitter; and the wireless power transmitter transmits, to the wireless power receiver, the at least one piece of ADT information in the power transmission phase on the basis of the information related to the unit size.