FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] WIRELESS COMMUNICATION APPARATUS AND RESPONSE SIGNAL SPREADING METHOD; PANASONIC CORPORATION, A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 1006, OAZA KADOMA, KADOMA-SHI, OSAKA 571-8501, JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. 1 DESCRIPTION Technical Field The present invention relates to a radio communication apparatus and response signal spreading method. Background Art In mobile communication, ARQ (Automatic Repeat Request) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to "base station") to radio communication mobile station apparatuses (hereinafter abbreviated to "mobile stations"). That is, mobile stations feed back response signals representing error detection results of downlink data, to the base station. Mobile stations perform a CRC (Cyclic Redundancy Check) of downlink data, and, if CRC=OK is found (i.e. if no error is found), feed back an ACK (ACKnowledgement), and, if CRC=NG is found (i.e. if error is found), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station. These response signals are transmitted to the base station using uplink control channels such as a PUCCH (Physical Uplink Control CHannel). Also, the base station transmits control information for reporting resource allocation results of downlink data, to mobile stations. This control information is transmitted to the mobile stations using downlink control channels such as L1/L2 CCH's (L1/L2 Control CHannels). Each L1/L2 CCH occupies one or a plurality of CCE's. If one L1/L2 CCH occupies a plurality of CCE's (Control Channel Elements), the plurality of CCE's occupied by the L1/L2 CCH are consecutive. Based on the number of CCE's required to carry control information, the base station allocates an arbitrary L1/L2 CCH among the plurality of L1/L2 CCH's to each mobile station, maps the control information on the physical resources corresponding to the CCE's (Control Channel Elements) occupied by the L1/L2 CCH, and performs transmission. Also, to efficiently use downlink communication resources, studies are underway to map between CCE's and PUCCH's. According to this mapping, each mobile station can decide the PUCCH to use to transmit response signals from the mobile station, from the CCE's mapped to physical resources on which control information for the mobile station is mapped. Also, as shown in FIG.l, studies are underway to perform code- multiplexing by spreading a plurality of response signals from a plurality of mobile stations using ZC (Zadoff-Chu) sequences and Walsh sequences (see Non-Patent Document 1). In FIG.l, (W0, W,3 W2, W3) represents a Walsh sequence with a sequence length of 4. As shown in FIG.l, in a mobile station, first, a response signal of ACK or NACK is subject to first spreading to one symbol by a ZC sequence (with a sequence length of 12) in the frequency domain. Next, the response signal subjected to first spreading is subject to an IFFT (Inverse Fast Fourier Transform) in association with W0 to W3. The response signal spread in the frequency domain by a ZC sequence with a sequence length of 12 is transformed to a ZC sequence with a sequence length of 12 by this IFFT in the time domain. Then, the signal subjected to the IFFT is subject to second spreading using a Walsh sequence (with a sequence length of 4). That is, one response signal is allocated to each of four symbols S0 to S3. Similarly, response signals of other mobile stations are spread using ZC sequences and Walsh sequences. Here, different mobile stations use ZC sequences of different cyclic shift values in the time domain or different Walsh sequences. Here, the sequence length of ZC sequences in the time domain is 12, so that it is possible to use twelve ZC sequences of cyclic shift values "0" to "11," generated from the same ZC sequence. Also, the sequence length of Walsh sequences is 4, so that it is possible to use four different Walsh sequences. Therefore, in an ideal communication environment, it is possible to code-multiplex maximum forty-eight (12x4) response signals from mobile stations. Here, there is no cross-correlation between ZC sequences of different cyclic shift values generated from the same ZC sequence. Therefore, in an ideal communication environment, as shown in FIG.2, a plurality of response signals subjected to spreading and code-multiplexing by ZC sequences of different cyclic shift values (0 to 11), can be separated in the time domain without inter-code interference, by correlation processing in the base station. However, due to an influence of, for example, transmission timing difference in mobile stations, multipath delayed waves and frequency offsets, a plurality of response signals from a plurality of mobile stations do not always arrive at a base station at the same time. For example, as shown in FIG.3, if the transmission timing of a response signal spread by the ZC sequence of cyclic shift value "0" is delayed from the correct transmission timing, the correlation peak of the ZC sequence of cyclic shift value "0" may appear in the detection window for the ZC sequence of cyclic shift value "1." Further, as shown in FIG.4, if a response signal spread by the ZC sequence of cyclic shift value "0" has a delay wave, an interference leakage due to the delayed wave may appear in the detection window for the ZC sequence of cyclic shift value "1." Therefore, in these cases, the separation performance degrades between a response signal spread by the ZC sequence of cyclic shift value "0" and a response signal spread by the ZC sequence of cyclic shift value "1." That is, if ZC sequences of adjacent cyclic shift values are used, the separation performance of response signals may degrade. Therefore, up till now, if a plurality of response signals are code-multiplexed by spreading using ZC sequences, a sufficient cyclic shift value difference (i.e. cyclic shift interval) is provided between the ZC sequences, to an extent that does not cause inter-code interference between the ZC sequences. For example, when the difference between the cyclic shift values of ZC sequences is 4, only three ZC sequences of cyclic shift values "0," "4," and "8" amongst twelve ZC sequences of cyclic shift values "0" to "11," are used for the first spreading of response signals. Therefore, if Walsh sequences with a sequence length of 4 are used for second spreading of response signals, it is possible to code- multiplex maximum twelve (3x4) response signals from mobile stations. Non-Patent Document 1: Multiplexing capability of CQIs and ACK/NACKs form different UEs (ftp://ftp.3gpp.org/TSG_RAN/WGl_RLl/TSGRl_49/Docs/Rl-0723 15.zip) Disclosure of Invention Problems to be Solved by the Invention As described above, if a Walsh sequ£nce with a sequence length of 4, (Wo, W|, W2, W3), is used for second spreading, one response signal is allocated to each of four symbols (S0 to S3)- Therefore, a base station that receives response signals from mobile stations needs to despread the response signals over a time period of four-symbols. On the other hand, if a mobile station moves fast, there is a high possibility that the channel conditions between the mobile station and the base station change during the above four-symbol time period. Therefore when there is a mobile station moving fast, orthogonality between Walsh sequences that are used for second spreading may collapse. That is, when there are mobile stations moving fast, inter-code interference is more likely to occur between Walsh sequences than between ZC sequences, and, as a result, the separation performance of response signals degrades. By the way, when some of a plurality of mobile stations moves fast and the rest of mobile stations are in a stationary state, the mobile stations in a stationary state, which are multiplexed with the mobile stations moving fast on the Walsh axis, are also influenced by inter-code interference. It is therefore an object of the present invention to provide a radio communication apparatus and response signal spreading method that can minimize degradation of the separation performance of response signals that are code-multiplexed. Means for Solving the Problem The radio communication apparatus of the present invention employs a configuration having: a first spreading section that performs first spreading of a response signal using