PURPOSE: To effectively listen to the alarm whistle of a facing car by automatically attenuating the acoustic output of an acoustic system in a constant period after an alarm whistle is operated.

CONSTITUTION: There are provided an input circuit INP in which an input end is connected to the connecting point of the coil CL1 of a relay RL1 for the alarm whistle having one end connected to a constant voltage source Vcc and the other end earthed through a depressing button-switch SW1 and the depressing button-switch and a first signal is outputted from an output end when the depressing button-switch is depressed, a timer circuit TM in which an input end is connected to the output end of the input circuit and a second signal is outputted from an output end when the first signal is inputted and an output circuit OUP in which an input end is connected to the output end of the timer circuit and an attenuation resistance is inserted to the speaker of the acoustic system AUD according to the second signal. Thereby, the attenuation resistance is automatically inserted to the speaker of the acoustic system by depressing the depressing button-switch of an alarming device to attenuate the acoustic output and make a driver listen to the alarm whistle of the facing car effectively.