Terminal device, base station device, communication method, and control method

The purpose of the present invention is to cause a terminal device and a base station device to efficiently sustain communication. Provided is a terminal device which carries out a handover from a source cell to a target primary cell, said terminal device comprising a receiving unit which receives a handover command, and a transmission unit which, after having commenced a synchronization with a downlink of the target primary cell, transmits a RRC complete message. If a parameter MobilityControlInfo which is included in the handover command includes a parameter which is associated with Time Advance or Time Alignment (TA), and if a PDCCH is detected in the target primary cell, with a CRC parity bit appended to said PDCCH, said CRC parity bit having been scrambled by a C-RNTI, a T304 timer is suspended.