A base station apparatus used in a mobile communication system to which an OFDM scheme is applied in downlink is provided with: a scheduler configured to perform assignment of radio resources to a user apparatus for each subframe; a control channel generation unit configured to generate a control channel for reporting a result of scheduling by the scheduler to the user apparatus; and a mapping unit configured to map the control channel and a data channel, wherein the control information includes information indicating a radio resource amount used for the control channel, and the mapping unit multiplexes information indicating the radio resource amount used for the control channel into a first OFDM symbol.