PURPOSE: To allow a supervisor to always monitor a video of all monitor cameras by overlappingly displaying a synthesized pattern of plural monitor camera videos onto a monitor video displayed in the size of a full screen.

CONSTITUTION: This device is provided with an image changeover means 113 selecting image data from plural monitor cameras 101-104 and with a superimposing means 115 superimposing each of image data onto image data outputted from an image synthesis means 114 synthesizing the image data. Even when the image data outputted from the image changeover means 113 are displayed in a size of a full screen of a display device 117 and supervised, the superimposing means 115 displays the synthesis image of all monitor cameras synthesized by the image synthesis means 114 superimposingly.