A wireless communication system comprising: a first base station device; a second base station device; and a mobile station device configured to communicate with the first and second base station devices (101, 102), characterized in that the first base station device comprises: a request notifying unit configured to notify the mobile station device of first information along with a request for carrier aggregation, the first information specifying at least one base station device targeted for a cell search, the at least one base station device being able to perform the carrier aggregation in combination with the first base station device, and the mobile station device comprises: a cell search unit configured to, upon receiving the request, perform the cell search to search at least one base station device specified by the first information notified by the request notifying unit; and a base station notifying unit configured to notify the first base station device of second information, the second information indicating the second base station device, the second base station device being one of at least one base station device detected by the cell search unit performing the cell search. a first base station device; a second base station device; and a mobile station device configured to communicate with the first and second base station devices (101, 102), characterized in that the first base station device comprises: a request notifying unit configured to notify the mobile station device of first information along with a request for carrier aggregation, the first information specifying at least one base station device targeted for a cell search, the at least one base station device being able to perform the carrier aggregation in combination with the first base station device, and a request notifying unit configured to notify the mobile station device of first information along with a request for carrier aggregation, the first information specifying at least one base station device targeted for a cell search, the at least one base station device being able to perform the carrier aggregation in combination with the first base station device, and the mobile station device comprises: a cell search unit configured to, upon receiving the request, perform the cell search to search at least one base station device specified by the first information notified by the request notifying unit; and a base station notifying unit configured to notify the first base station device of second information, the second information indicating the second base station device, the second base station device being one of at least one base station device detected by the cell search unit performing the cell search. a cell search unit configured to, upon receiving the request, perform the cell search to search at least one base station device specified by the first information notified by the request notifying unit; and a base station notifying unit configured to notify the first base station device of second information, the second information indicating the second base station device, the second base station device being one of at least one base station device detected by the cell search unit performing the cell search.