PURPOSE: To adjust a sound pressure level quickly and simply by providing a ROM storing a sinusoidal wave digital data to the processing unit, selecting the digital data in place of an audio data as required and applying arithmetic operation processing to the selected data so as to obtain a referenced prescribed signal.

CONSTITUTION: An input audio data is extracted by an interface circuit 3a and outputted via a filter circuit 3b and a D/A converter 3c when a switch 1e is thrown to the position of A. When the sound pressure level is desired to be adjusted, the switch 1e is thrown to the position B, a reference signal is obtained from a ROM 3f into which a prescribed sinusoidal digital data is stored to adjust an output variable resistor of a power amplifier 4 so as to obtain a desired sound pressure. Thus, in the case of adjustment of the sound pressure level, it is not required to obtain a reference signal from a pattern generator on each occasion and quick and simple adjustment is implemented.