The present invention relates to a wireless communication system. More specifically, the present invention relates to a method for transmitting data by a terminal in a wireless communication system supporting high-speed uplink transmission, the method comprising the steps of: transmitting or receiving a signal to or from a serving cell by a terminal in a state where the terminal is in radio resource control (RRC) connection with the serving cell; performing measurements for the serving cell and a neighboring cell; and transmitting data to the neighboring cell on the basis of the measurements in a state where RRC connection with the serving cell is established.