To prevent the siren of an emergency vehicle and the sound of a railroad crossing signal from not being heard at the time of listening to a car stereo inside a car.

When the siren 11 of an emergency vehicle approaches, the ratio of the siren sound to sounds that are picked up by a sound collecting means 1 becomes higher. Because of this, in a correlation detecting means 5, when the correlation degree of a signal that is digitally encoded by a CODEC 3 to the pattern of a siren which is preliminarily stored in memory 4 rises gradually and reaches a certain correlation degree or more, the means 5 decides that the signal from the CODEC 3 is the sound of a siren and outputs a decision signal of correlation existence to a level regulator 7. The regulator 7 receives it, turns the level of a music source 6 down (lowers) and outputs it and as a result the volume of a speaker 9 drops.