The present disclosure provides 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: receiving, from a wireless power receiver, information for sensing whether a foreign object (FO) is present; sensing, on the basis of the information, whether the FO is present before the transmission of wireless power; transmitting, on the basis of sensing that the FO is not present, the wireless power to the wireless power receiver and measuring a slot quality factor and a coupling factor on a slot while the wireless power is transmitted, wherein the slot quality factor is a quality factor used to determine whether the FO is inserted while the wireless power is transmitted, and the coupling factor is a factor related to the alignment between the wireless power transmitter and the wireless power receiver.