The present invention relates to a method by which a terminal transmits/receives data to/from a first base station (BS) supporting a first radio access technology (RAT) and a second BS supporting a second RAT in a wireless access system supporting multi-RAT, the method comprising the steps of: executing an initial network entry to the first BS; a step for executing a multi-RAT capability negotiation with the first BS to exchange information needed to access the second BS; executing a scanning on at least one second BS included in the coverage of the first BS, on the basis of the information received from the first BS through the multi-RAT capability negotiation process; executing an access to a second BS of the at least one second BS; and simultaneously transmitting/receiving data with the first BS and the second BS accessed through the access process.