In TDD-FDD carrier aggregation, delay in downlink retransmission timing and UE's battery energy consumption are reduced. In a mobile communication system that performs carrier aggregation using a first component carrier associated with a first cell conducting radio communication in a first duplex mode and a second component carrier associated with a second cell conducting radio communication in a second duplex mode, a cell is provided on which an uplink control channel carrying acknowledgement or negative acknowledgement for the first component carrier and the second component carrier is transmitted, and a user equipment performs discontinuous reception using a timer value corresponding to a duplex mode of said cell on which the uplink control channel is transmitted upon receiving at least one of a first downlink data signal transmitted over the first component carrier and a second downlink data signal transmitted over the second component carrier.