PURPOSE: To grasp easily the information of the arrival time of an earthquake wave and its classification, etc. by indicating clearly the waveform of a detecting signal by a vertical straight line at the display position of a start time point of its amplitude variation, in case of displaying the waveform of the detecting signal, and also displaying its amplitude direction by the display of a vertical arrow.

CONSTITUTION: Sensors S1WSn placed in each place are connected to a center 2 by a circuit 1. For instance, when the sensor S1 detects an earthquake in the vicinity of the sensor S1, a detecting signal I1 is received by a receiving part 3, and sent to a sampling circuit 4 and a display control device 5. A detecting signl D is outputted from the sampling circuit 4, and it is sent to a discriminating part 6. When the first sampling signal d1 of the detecting signal D is received, the discriminating part 6 generates a set signal F, and sets a time data T0 of a time counter 7 to a register 8. Also, the discriminating part 6 discriminates the polarity of the first sampling signal d1 of the detecting signal I1, generates a positive signal P and a negative signal M in case of positive and in case of negative polarity respectively to display on a display part 14.