PURPOSE: To execute the supervision/operation of individual devices on an entire system diagram by dividing a virtual image into an individual system block and a detailed block and displaying a block window on the entire system diagram as required.

CONSTITUTION: A processing section 1 has a system operation processing function and a graphic processing function or the like and has an installation system diagram with plotted data far larger than a screen of a CRT 2 as a virtual image, which is divided into partial blocks. When simultaneous confirmation is required on the entire system in the supervision/operation of individual devices, a mouse is used to select an area in the vicinity of any device on a reduced entire system diagram displayed on the CRT 2. The selection signal is introduced to the graphic processing function, by which a corresponding part is segmented from the virtual image in a fom of a block window and displayed on the reduced entire system diagram. The detailed block for the block window is moved and magnified/reduced in an optional size and the individual system diagram in the window is displayed in a scrolling way.