A user apparatus for use in a mobile communications system is disclosed. The user apparatus includes a unit which determines, using acknowledgement information received from a communications counterpart, whether retransmission is required for a transmitted signal; and a unit which transmits, to the communications counterpart, a signal required to be retransmitted according to a round-trip delay (RTD) which is set for the user apparatus for retransmission control, wherein, the RTD of the user apparatus is arranged to be changed, when the user apparatus enters a region which uses an RTD shorter than the RTD set for the user apparatus to an integer multiple of the shorter RTD.