The present invention can adjust a volume of a sound on the basis of sound signals to suit a user's preference.The present invention arbitrarily sets movement settable control values application range DR2 for a plurality of continuous preamp control values PD from a desired minimum preamp control value PD to a desired maximum preamp control value PD against a group of preamp control available values PDS. Thus, the present invention can control gains so as to compensate external sound signals with different signal levels supplied from external input terminal OI for difference of signal level according to the external sound signal or can control gains in a predetermined range according to the signal level so that the volume of the external sound on the basis of the external sound signals can be arbitrarily adjusted in a range between a desired minimum value and a desired maximum value. In this manner, a volume of a sound on the basis of an external sound signal can be adjusted.