Dislosed is a fast Fourier transform device of a wireless LAN orthogonal frequency division multiplexing (OFDM) system. The device comprises: an FFT/IFFT unit constituted with one hardware, for performing a fast Fourier transform (FFT) and an inverse fast Fourier transform (IFFT); and a control signal input unit for outputting a control signal for controlling an operation of the FFT/IFFT unit.