PURPOSE: To monitor a crack and deformation at the time of an earthquake by operating an ultrasonic surface wave sensor provided at the lower part of a tank side plate and the connection of an annular plate, etc., through an accelerometer on a storage tank foundation and by collecting and recording obtained data.

CONSTITUTION: The side plate 6 of a storage tank 3 and the connection 8 of an annular plate 5 are provided with an ultrasonic surface wave sensor 9 for detecting the deformation and crack of sand parts and an ultrasonic wave is oscillated normally from the transmitting sensor 9a of that sensor and received by a receiving sensor 9b and the waveform of a received signal is recorded as the state of the connection 8 on a collecting and recording apparatus 13. At the time of the occurrence of an earthquake, an accelerometer 11 on a foundation 1 detects the earthquake and when the acceleration of the earthquake is higher than a certain level, a starter 14 operates the surface wave sensor 9 via the collecting and recording apparatus 13. When a crack occurs in the connection 8, the signal received by the receiving sensor 9b changes relative to an initial waveform signal and the collecting and recording apparatus 13 can read this change and monitor in what position of the connection 8 a crack, etc., occur.