A communication system in which a base station apparatus and a user equipment communicate on multiple component carriers comprising primary component carrier and non-primary component carrier, the base station apparatus, transmits, to the user equipment, four indices for physical uplink control channel resource using higher layer signaling, transmits, to the user equipment, an index indicator using transmission power control command for physical uplink control channel field, the user equipment, determines an index among the four indices according to the index indicator, transmits, to the base station apparatus, HARQ-ACK/NACK using physical uplink control channel resource the index, wherein the transmission power control command for physical uplink control channel field is included in a downlink control information format, which is used to schedule a physical downlink shared channel on the non-primary component carrier.