To prevent an excess burden on one's ears by adding reverberation components to an acoustic signal and adjusting a sound pressure level by taking the psycho-logical volume of sound into consideration.

The acoustic signals from a monophonic signal source 1 are supplied to reverberation sound addition means 2 L and 2 R via an input means 1a. In the means 2L and 2R, the reverberation components are prepared from the acoustic signals, these reverberation components are added to the acoustic signals and the signals are supplied to output level adjustment means 3L and 3R. In the means 3L and 3R, the voltage levels of the inputted signals are adjusted and the levels are sent to an output means 1b. In the means 1b, the acoustic signals are supplied to a headphone.