A Fast Fourier Transform (FFT) apparatus for selectively performing Fast Hadamard transform (FHT), and a complementary code keying (CCK) modulation/demodulation apparatus using the same are disclosed. In the present invention, OFDM module and CCK module are integrated as one module having lower complexity compared to conventional scheme by embodying the CCK modulation using FFT structure in the OFDM module, and embodying the suboptimal and the optimized CCK modulations using FFT structure in OFDM module. Furthermore, according to the present invention, CDMA, OFDM, CCK modules may be integrated as single module.