There is provided a method for accurately compensating a DC offset component contained in a signal to be modulated, i.e. a transmission signal generated in a quadrature modulation system. A DC offset correction value determined from the transmission signal is subjected to weighting in accordance with the signal level of an input signal to the quadrature modulation system as transmission data, and then the DC offset component contained in the transmission signal is compensated based on the weighted DC offset correction value.