User equipment, and delivery-confirmation-information transmission method

User equipment that communicates with a base station over a mobile communication system that supports carrier aggregation configured from a plurality of cells including a first cell and a second cell that uses a TTI length different from the TTI length of the first cell. The user equipment is provided with: a reception unit that uses the second cell to receive downlink data transmitted from the base station, and that generates delivery confirmation information for the downlink data; and a transmission unit that bundles a plurality of items of delivery confirmation information generated by the reception unit for a plurality of sets of downlink data into a single item of delivery confirmation information, and that uses the first cell to transmit the bundled delivery confirmation information to the base station.