PURPOSE: To measure the state of cracking by detecting the amplitude attenuation rate of the elastic waves in the part of bedrock to be measured.

CONSTITUTION: First, a transmission vibration 1 and a reception vibrator 2 are pressed by load P, and elastic waves are applied to the vibrator 1 by a pulse generator 3, thence the amplitude A0 of the vibrator 2 is measured. Next, a part 4 of prescribed bedrock is provided between the vibrator 1 and the vibrator 2, and the amplitude A1 of the vibrator is similarly measured. Next, the difference A between the A0 and the A1 is determined, and the amplitude attenuation rate A of elastic waves is detected, whereby the state of cracking is judged. If the cracking is large, the rate A increases and if small, it decreases and therefore, the cracking state of said bedrock is judged if the amplitude attenuation rate of the elastic waves is measured.