PURPOSE: To improve the operability, by attenuating the level of a frequency region of an audible signal corresponding to a logical signal in response to the logical signal, by a value corresponding to the logical signal.

CONSTITUTION: An audible signal is amplified at a buffer 97 via a line 93 from a sound volume control circuit 5 in a sound quality control circuit 6, and given to an amplifying circuit 99 from a line 98. This line 98 is provided with filter circuits 101∼105 corresponding to frequency bands, 50, 250Hz, 1, 4, 16kHz. In the filter circuits 101∼103, the attenuation around each frequency band can be changed by selectively conducting transistors 107∼110 with a control circuit 106. In the filter circuits 104 and 105, transistors 117∼119 are selectively conducted in response to a digital signal from a bus 95, to change the attenuation around 4.16kHz of the audible signal.