Provided is a technique for specifying a secondary cell, to which a random access channel is to be transmitted, when a random access procedure for the secondary cell has occurred in user equipment. A mobile communication system including a first base station, a second base station, and user equipment connected with the first base station as a main base station, wherein when a random access procedure for a cell which the second base station uses is started in the user equipment, the user equipment performs a contention based random access procedure on a secondary cell in accordance with any method of (a) selecting the secondary cell, to which a random access signal is to be transmitted, in accordance with an instruction from the first base station, (b) selecting the secondary cell, to which the random access signal is to be transmitted, in the user equipment using implicit information, or (c) autonomously determining the secondary cell, to which the random access signal is to be transmitted, in the user equipment.