Disclosed are a terminal, a base station, a response method, and a retransmission control method, which are capable of switching the response signal transmission method while preventing throughput degradation and without increasing the number of signaling bits. A bundling determination unit (132) of a terminal (100) determines parameters that denote the allocated resource quantities that are allocated thereto from a base station (200), or in other words, a downstream band that is included in a bundling group and a downstream band that is not included in the bundling group, from among a plurality of downstream bands on the basis of base station transmission parameters. Specifically, a degree of resource share (Xn) that is allocated from each respective downstream band to the terminal (100) is employed in the base station transmission parameters.