PURPOSE:To obtain an operation log data of a plant, which has a high reliability, by providing a timer for ticking away the time, a device for detecting the start and stop of a computer, a device for storing a stop time and a plant input signal of a computer, etc., on the outside of a computer. CONSTITUTION:A signal from each place of a plant is detected by a pulse detector 12a and an analog detector 12b, and is inputted to an input signal memory device 13. An input signal which has been stored in the device 13 is controlled by an external controller CT14 and is inputted to a computer 15. The computer 15 calculates various data based on these signals and outputs a log 16. When the computer 15 stops, a start and stop detecting device 17 detects it and informs it to the CT14. When it is received, the CT14 reads a time of an external timer 19, and stores it in a stop time memory devie 18. When the computer starts, the device 17 detects it, sends a start detection signal to the CT14, and as a result, the CT14 sends a stop time and a start time of the computer, and an integrating data in the course of stop, to the computer 15, from the device 18, the timer 19 and the device 13, and the computer calcultes a log in the course of stop and outputs it.
