An apparatus and method for detecting a GCL sequence using a magnitude and a sign of a real part of a I/FDFT output except an imaginary part in a wireless communication system. The apparatus includes: a modified inverse/forward discrete fourier transform (I/FDFT) processing unit for enabling that the GCL sequence class u is detected with only a real part; a real value selecting unit for selecting real parts from the output of the I/FDFT processing unit; and a maximum value detecting unit for detecting a maximum value among magnitudes of the selected real parts having a positive sign from the real value selecting unit.