PURPOSE: To suitably display a message concerning the generation of abnormality by displaying the abnormal message on a pop-up window which is displayed by being superimposed on a monitoring screen.

CONSTITUTION: The abnormal message is displayed on the pop-up window, which is displayed by being superimposed on the display screen of a CRT display device 1, and the size of the pop-up window is determined based on the information quantity of the abnormal message to be displayed. Further, the display position of the pop-up window is determined based on input information from an input means 7 which constitutes an interacting means together with the CRT display device 1. Thus, it is not necessary to switch the display picture of the CRT display device and plant monitoring work can be smoothly executed. Then, the abnormal message with the enough information quantity can be displayed and an operator can recognize the abnormal message without fail.