PURPOSE: To quickly display the circumference of an optional point on the screen of a whole system with the optional point as a center by the optional magnification of enlargement only by changing the position of a finger tip by deciding the center of display and the magnification of the enlargement based on the position of a finger with respect to a display surface in the monitoring device of a plant system.

CONSTITUTION: The position of the finger 14 with respect to a CRT screen when the instruction is required, that means, coordinates X, Y and Z are detected by a finger position detection part 1. It is decided by a display center calculation part 2 based on the coordinates X and Y which part of the screen of the whole system is set as the center of the CRT display surface. Besides, the magnification of the enlargement given by the coordinate Z and the function of the magnification of the enlargement P=f(z) is obtained by an enlargement magnification calculation part 3. Based on the obtained center of the display and the obtained magnification of the enlargement, the display screen is edited from the screen of the whole system by a screen editing part 4 and the display screen is displayed and outputted by a display output part 5.