PURPOSE: To obtain an abnormality detector capable of accurately discriminating the abnormality of a knocking sensor even when irregularity is generated in a knocking sensor signal or even within a low rotation region where the number of rotations of an engine is Ne.

CONSTITUTION: The max. value (knocking intensity value V) of the predetermined section of the output signal from a knocking sensor 7 is calculated and the value [logarithmic conversion value LOG (V)] obtained by logarithmically converting said max. value is also calculated. Then, the central value VM of the distribution of the logarithmic conversion value LOG (V) and the index S (standard deviation σ) showing the degree of the irregularity of said distribution are renewed. Next, the index S is compared with an abnormality judging value Slh. By this constitution, since the irregularity of the sensor signal is reduced when abnormality is generated in the knocking sensor 7, by judging that the knocking sensor is abnormal when the index S is smaller than the judging value Slh, the abnormality of the knocking sensor 7 can be accurately judged.