A base station apparatus is used in a mobile communication system in which at least a shared data channel is transmitted by an orthogonal modulation scheme using an I-channel and a Q-channel. The base station apparatus includes a providing unit providing Acknowledgement/Non-Acknowledgement information indicating whether retransmission of a signal received in uplink is required; a mapping unit mapping a number of multiplexed users of the Acknowledgement/Non-Acknowledgement information to the I-channel or the Q-channel based on a predetermined mapping table; and a reporting unit reporting the Acknowledgement/Non-Acknowledgement information to the respective users. The mapping table indicates that, when the number of the multiplexed users is equal to or less than a predetermined number, the Acknowledgement/Non-Acknowledgement information is associated with the I-channel or the Q-channel, and otherwise, the Acknowledgement/Non-Acknowledgement information is associated with the I-channel and the Q-channel.