A mobile communication system and a communication method are provided in which, a base station apparatus can efficiently allocate, to a mobile station apparatus, an uplink resource for transmitting information indicating an ACK/NACK for downlink data. The mobile station apparatus that communicates with the base station apparatus, comprising: a scheduling unit configured to determine two physical uplink control channel resources for a transmission using a physical downlink shared channel in a case that a downlink transmission mode that supports the transmission of up to two downlink transport blocks using the physical downlink shared channel is configured for the secondary cell, and a physical downlink control channel that indicates the transmission using the physical downlink shared channel on the secondary cell is detected on the secondary cell.