PURPOSE: To enable automatic and accurate detection of a slack of a drilling rod when it looses, by striking the drilling rod with a rock drill to compare the striken sound with reference information to determine whether the characteristic thereof changes or not.

CONSTITUTION: A sound picked up with a microphone 8a is amplified with a sound input section 8 to be fed to a parameter extracting section 9. Then, first, a plurality of band-pass filters 11aW11n different in the band are used to single out a frequency spectrum comprising band-wise component as obtained by rectifying with rectifiers 12aW12n and integrating outputs of the filters. Powers of the components of the frequency spectrum are extracted and a spectrum normalizing section 14 determines differences between the powes of these components and the mean of power values in all channels to normalize a spectral power, which is used as feature parameter for collation. A vibration sound characteristic judging section 20 collates the resulting feature parameter with a reference feature parameter previously registered in a parameter memory section 21 to check changes in the characterisic of striken sounds of rods thereby enabling the detection of the slack of rods.