PURPOSE: To enable adjustment to a prescribed level without converting a digital audio signal to an analog signal by setting a gain based on the result of comparing the level of the digital audio signal with a set threshold value, and amplifying the digital audio signal to the prescribed level.

CONSTITUTION: An amplifier circuit 2 is provided to amplify the level of the digital audio signal, a gain setting part 3 is provided to set the gain of this amplifier circuit 2, a level detection circuit 4 is provided to detect the average level of the digital audio signal, and a comparator circuit 6 is provided to compare the average level of the digital audio signal with the threshold value set to a threshold value setting register 5. The gain is set based on the compared result of this comparator circuit 6, and the digital audio signal is amplified to the prescribed level by the amplifier circuit 2. Thus, the digital audio signal can be adjusted to the prescribed level without being converted to the analog signal.