PURPOSE: To easily switch the display screen of the operating states of respective plants and to enlarge the using method of the display screen by providing a picture signal switching means for switching picture signals from plural picture signal generation means and supplying them to a display means.

CONSTITUTION: Plant state signals are sent out through a communication controller 4 and a line 3 to picture signal generators 2a and 2b by a computer 8 and the output signals are inputted through a switching device 9 to a video projector 1. The respective plant state signals inputted to the picture signal generators 2a and 2b may be same signals or different signals. When the same signals are inputted, by switching the output signals of the picture signal generators 2a and 2b by the switching device 9, the picture signal generators 2a and 2b are operated mutually as backup. When the different signals are inputted to the picture signal generators 2a and 2b, the display screen of the video projector 1 can be switched at a high speed by the switching device 9.