A user apparatus configured to perform communication with a base station in a mobile communication system that supports downlink spatial multiplexing, including: transmission means configured to transmit capability information of the user apparatus to the base station; reception means configured to receive configuration information from the base station; bit width determination means configured, if a parameter indicating a transmission mode corresponding to the downlink spatial multiplexing and predetermined additional information are received as the configuration information, to determine a bit width of a rank indicator based on the predetermined additional information.