PURPOSE: To provide a system monitor device which easily and speedily recognize the state of the total change of an object to be monitored and improves the safety and reliability of system monitoring by recognizing current values of plural state quantities and their tendencies at the same time.

CONSTITUTION: An input part 2 inputs detection data at specific timing by detectors 1a, 1b...1z and sends the data to a processing part 3. The data processing part 3 compares last input values (current display values) stored in a data storage part 4 with the current input values to find their increments and decrements and decides whether the increments and decrements are larger than a previously set threshold value. Then mutually difference command signals are sent to a display control part 5 according to whether the increments exceed the threshold value, whether the decrements exceed the threshold value, and other cases, and displays in different modes are made at a display part 6.