In an LTE-A system, there is a problem that it is difficult to perform stable communication in a case where there is a time when another system occupies an unlicensed band being used. There is provided a base station apparatus that communicates with a terminal device in a second frequency band different from a first frequency band capable of being used as a dedicated frequency band. The base station apparatus includes: a wireless transmission unit that transmits data or control information to the terminal device; and a wireless reception unit that receives data or control information transmitted from the terminal device. The wireless transmission unit transmits data to the terminal device, and the wireless reception unit performs a process of receiving ACK/NACK transmitted from the terminal device in a subframe of an initial uplink after communication of another system is ended in a case where the other system performs the communication in the second frequency band at a reception timing of the ACK/NACK of the data transmission.