A user apparatus configured to communicate with a base station in a mobile communication system that supports carrier aggregation formed by a plurality of cells including a first cell and a second cell that uses a unit time length different from a unit time length of the first cell, including: a reception unit configured to receive downlink data transmitted from the base station in the second cell, and generate transmittal acknowledgement information for the downlink data; and a transmission unit configured to bundle a plurality of pieces of transmittal acknowledgement information, generated in the reception unit, for a plurality of pieces of downlink data into one piece of transmittal acknowledgement information to transmit the bundled transmittal acknowledgement information to the base station in the first cell.