A terminal apparatus and a response signal transmitting method wherein the system transmission efficiency can be improved by devising a bundling rule. In a terminal (200), a PDSCH receiving unit (208) receives first and second code words, which comprise code word block (CWB) groups to be mapped to the respective ones of a plurality of downstream unit bands, and detects errors of the CWBs; and an A/N bundling unit (216) transmits, based on a bundling rule in which each CWB is associated with any one of first and second bundling groups, a single bundle of response signals into which the error detection results are bundled for each bundling group. According to the bundling rule, at least one of the first and second bundling groups includes both the CWB belonging to the first code words and the CWB belonging to the second code words.