PURPOSE: To detect liquefaction of the ground quickly while eliminating the probability of ineffective decision by sensor at the time of earthquake and then grasping the conditions of the ground using an AE sensor.

CONSTITUTION: Upon occurrence of earthquake, an acceleration sensor 3 detects occurrence of earthquake and the state of quake and delivers signals to a personal computor D through an amplifier B while, at the same time, an AE sensor 4 is actuated to measure the generation trend of AE in the ground A and deliver a signal to the personal computor D. When the ground is not liquefied, friction takes place between particles in the ground A and detected by the sensor 4 and a decision is made that the ground is not liquefied if generation of AE increases as the quake increases. On the contrary, when the hydraulic pressure in the gap of the ground A increases to cause liquefaction, the frictional resistance between particles decreases to reduce the frictional noise. A decision is made that the ground is liquefied at a moment of time when generation of AE decreases abruptly with respect the quake of the ground A.