In a mobile station that performs communication with a base station device via a plurality of cells, the plurality of cells are grouped into a first group constituted by cells having an uplink transmission timing that is the same as that of one primary cell and a second group constituted by one or more cells having an uplink transmission timing that is different from that of the primary cell. The uplink transmission timings are each managed using a corresponding transmission timing timer for each of the groups. In a case where a transmission timing timer for the first group has expired, the mobile station stops all uplink transmissions in the plurality of cells. In a case where a transmission timing timer for the second group has expired, the mobile station stops uplink transmissions in the cells constituting the second group for which the transmission timing timer has expired.