In the specification, a method for a terminal to transceive data to/from a first base station supporting a first RAT and a second base station supporting a second RAT in a radio access system supporting multi-Radio Access Technology (RAT) includes: performing a data communication on a first service flow with the first base station; receiving a first message, which instructs an access of the terminal by the second base station, from the first base station in order for the terminal to perform a data communication on a second service flow with the second base station; performing a procedure of accessing the second base station; and performing a data communication on the second service flow with the second base station according to the run-time information.