PURPOSE: To provide a detector capable of detecting strength of knocking accuracy.

CONSTITUTION: A signal from a knocking sensor 8 detecting knocking of an engine is A/D converted by an A/D converter 14 each are determined time, and thereafter frequency is analyzed by a digital signal processor 13. Plural components of frequency is compared with a predetermined threshold value based on a result of the frequency analysis by the digital processor 13. Zero is substituted if the result is lower than the threshold value, and each frequency component is added for the detection of knocking strength. A threshold value to each frequency component is stored respectively. An examination in a maximum of each frequency component is reflected in a central value of retrieval frequency.