1. An apparatus for compensating for an I (in-phase)/Q (quadrature-phase) mismatch in a wireless communication system, the apparatus comprising: a detector for detecting transmission signals and providing the transmission signals to a reception path; a Fast Fourier transform (FFT) operator for performing FFT on a detected transmission signal; and an I/Q mismatch estimator for estimating the I/Q mismatch using a preamble of the transmission signal after the FFT, wherein the detector detects the transmission signals in transmission time sections during which no signals are received, and wherein the detected transmission signal includes the preamble, wherein the I/O mismatch estimator calculates compensation coefficients, a phase mismatch, and a gain mismatch using subcarriers of the Fourier-transformed preamble, and an image signal of the subcarriers, and wherein the subcarriers of the Fourier-transformed preamble and the image signal of the subcarriers are calculated by: R R ⁡ ( kB ) = C k 2 ⁢ ( 1 + ⅇ - cos ⁢ ⁢ φ + j ⁢ ⁢ sin ⁢ ⁢ φ ) ⁢ exp ⁡ ( jθ ) R r ⁡ ( ( K - K ) ⁢ B ) = C K - k 2 ⁢ ( 1 + ⅇ - cos ⁢ ⁢ φ + j ⁢ ⁢ sin ⁢ ⁢ φ ) ⁢ exp ⁡ ( - jθ ) where R r (kB) is a subcarrier of a Fourier-transformed preamble R r ((K-k)B) is an image signal of the subcarrier, C k is a coefficient of the subcarrier, “e” is gain mismatch, φ is phase mismatch, θ is a phase of the oscillator 321 , and K is a length th of Fourier transform.