A signal generator generates a test signal including a positive signal and a negative signal which have the same amplitude. The signal generator corrects a DC level of the test signal based on a DC offset correcting signal supplied thereto, and supplies the corrected test signal to a frequency converter. An amplitude detector detects the amplitudes of the positive and negative signals of the test signal processed by the frequency converter. A level compressor converts in level the amplitudes of the positive and negative signals which are detected by the amplitude detector, with a gain variable depending on an input level thereto. A comparator compares the amplitudes of the positive and negative signals which are converted in level by the level compressor, with each other. An offset adjuster supplies the DC offset correcting signal depending on a compared result from the comparator to the signal generator.