PURPOSE: To output commands corresponding to the degrees of earthquakes by storing beforehand the natural oscillation frequencies and amplitude data of the respective apparatus included in plant systems, and analyzing said information and the actually measured data of the earthquakes.

CONSTITUTION: The natural oscillation frequencies and amplitude data of respective apparatus such as computers, control devices and the like included in plant systems are beforehand stored in an apparatus oscillation information table 8 in a processor 4. In this state, the acceleration detectors 1W3 disposed to all the apparatus groups output the DC voltage values proportional respectively to the magnitudes of the oscillations in the detection directions at all times and are supplying the same to the unit 4. In the unit 4, the operations of the signals inputted from the detectors and the comparative operations with the data of the table 8, and whether the oscillations giving danger to the apparatus are generated or not are judged. According to the permissible seismic levels of the apparatus, an alarm command 11 or a system stop command 12 is outputted from a digital output device 10.