To improve the accuracy of glass cracking detection.

A microphone 1 transduces a glass cracking sound into an electric signal and HPFs 3 and 13 extract high-frequency components of ≥2kHz and 15 Hz from the electric signal. Then a half-wave rectifying circuit 11 and an amplifier 2 process those signals by half-wave rectification and amplification. The output of the amplifier 2 is smoothed by smoothing circuits 51 and 52 and when the output of the smoothing circuit 51 reaches a specific value, a trigger circuit 12 outputs an actuation pulse to the actuation terminal of a CPU 10. The CPU 10 is actuated with the actuation pulse to inputs the outputs of the smoothing circuits 51 and 52, an A/D converter converts them into digital signals, and integral values in 30msec are found and stored in a RAM, thereby calculating the ratio of the integral value of the smoothing circuit 51 to the integral value of the output of the smoothing circuit 52 according to a program stored in a ROM. When the value of the ratio is within a specific range, it is decided that the detected sound wave is the 1st wave of the glass cracking sound and a glass cracking detection signal 7 is outputted to the outside.