In a mobile communication system including a first base station, a second base station, and user equipment connected to the first base station as a master base station, upon triggering a random access procedure toward a cell of the second base station, the user equipment performs a contention based random access procedure on a secondary cell, based upon one of (a) selecting the secondary cell on which a random access signal is to be transmitted in accordance with an instruction from the first base station; (b) selecting the secondary cell on which the random access signal is to be transmitted using implicit information in the user equipment; and (c) determining autonomously at the user equipment the secondary cell on which the random access signal is to be transmitted.