PURPOSE: To continuously fetch and store display data at the most excellent timing and to easily reproduce it later on by fetching one displayed data of a display device as a trigger signal and specifying the fetching timing of the other display data and a write address in a memory by using it.

CONSTITUTION: In the case that the data displayed on the liquid crystal display devices 3b and 3c of the display device 1 is monitored, the display data (b) is automatically written at the new address of the data memory 11 at the most excellent timing by turn when a fetching switch 12 is turned on. Next, the display data (a) which becomes an optimum reference among the respective liquid crystal display devices 3a, 3b and 3c is regarded as the trigger signal (a) showing the fetching timing. Thus, the data is always fetched at the correct timing without specifying the fetching timing of the respective data by the key operation of an operator. Besides, the data (b) stored in the memory 11 is reproduced and displayed on a monitor display device 18 by the simple operation.