In ARQ where a plurality of mobile stations share and use a single channel for response signals (ACK/NACK signals), a wireless communication base station apparatus can prevent failure of the ARQ control. In this apparatus, a CRC part (114) performs an error detection using CRC for an uplink data, and generates, as a response signal, an ACK signal in a case of CRC=OK or a NACK signal in a case of CRC=NG. A retransmittal determining part (115) determines whether the response signal is for an initially transmitted data or for a retransmitted data. A constellation control part (116) controls, in accordance with a determination result of the retransmittal determining part (115), a constellation pattern to be used in a modulation in a modulating part (105). The modulating part (105) modulates the response signal for the retransmitted data in accordance with the constellation pattern controlled by the constellation control part (116), that is, a constellation pattern obtained by reversing the constellation pattern of the response signal for the initially transmitted data.