If a display mode is set to a narrow angle enlargement image mode, an enlargement image area is inputted via a touch panel. A transformation is made to compress an image of a not-enlargement image area other than the enlargement image area, and a narrow angle enlargement image is generated to enable simultaneously displaying, as a narrow angle enlargement image, (i) the enlarged image in the enlargement image area and (ii) the compressed image in the not-enlargement image area. When a warning target is detected, a corresponding image area is enlarged. An audio alarm is outputted via a speaker. Further, a new narrow angle enlargement image is generated so as to merge the initial narrow angle enlargement image with an image of enlarging the image area of the warning target. A display window including the new narrow angle enlargement image is then displayed in a display device.