A test signal, the amplitudes of which are equal in the positive and negative directions, is generated and supplied to a signal processing circuit that performs a frequency conversion. Signals outputted from the signal processing circuit are detected to provide detected signals, which comprise a detected positive signal corresponding to a positive signal of the test signal and a detected negative signal corresponding to a negative signal of the test signal. The level of the detected positive signal is compared with the level of the detected negative signal to output a comparison result that indicates which level is higher. Additionally, an offset correction signal, which causes the level difference between the detected positive signal and the detected negative signal to be within a predetermined range of tolerance, is generated based on the comparison result. Then, an offset correction of the test signal and an externally supplied modulated signal is performed in accordance with the offset correction signal.