PURPOSE: To enable clear recognition of the voltage stability index by calculating the calculation result of the voltage stability index and the control effect amount of various kinds of control device from the three-dimensional data of a selected screen to three-dimensionally display the relatio of three indexes.

CONSTITUTION: A screen display processing means 40 is so constituted as to arbitrarily set and input three parameters to be displayed, a display color and a display method from some kinds of three-dimensional items to a data selection processing means 40-1 from a input device 30 or to select the items in which the display method and the display color are predetermined with three parameters. The coordinate for three-dimensionally displaying three parameters in a screen on a plane is calculated by a three-dimensional display calculation processing means 40-2 to the selected content and is stored into a three- dimensional display data file 40-3, and three dimensional display is represented by a three-dimensional display processing means 40-4 and displayed in the screen of VDU 50. As a result, the voltage stability is clear at a glance from the shape and size of a figure which is displayed with the three-dimensional display in which three voltage stability indexes are used as parameters.