1. An accident sound detection circuit for detecting an accident sound by using a sound signal produced by an environmental sound detection unit located in the vicinity of a road, the accident sound detection circuit comprising: an analog-to-digital (A/D) converter for converting a sound signal from an environmental sound detection unit into a digital sound signal; differential calculation means for generating an acquired differential signal from an output signal of said A/D converter; a reference differential value generating circuit for generating a reference differential value; a first comparator circuit for comparing the acquired differential signal to the reference differential value, detecting an accident sound from a comparison of the acquired differential signal to the reference differential value, and generating an accident sound detection signal in response to detecting an accident sound; and auto-correlation calculation means for auto-correlation of the digital sound signal received from the A/D converter when an accident sound detection signal is input; a correlation reference value generating circuit for setting an auto-correlation reference value of a sound signal for discriminating among different kinds of accident sounds; and a second comparator circuit for comparing the auto-correlation reference value and the auto-correlation of the digital sound signal and for generating a final accident sound detection signal in response to the comparison of auto-correlation of the digital sound signal and the reference value.