Photographed images from a front camera and a side camera respectively installed at the front and left side of a vehicle are respectively converted into bird's-eye view images, and a synthetic bird's-eye view image is displayed which is obtained by synthesizing the bird's-eye view images. Based on a difference image between the front and side cameras, a difference region where a solid object is drawn is detected from a common region where the both bird's-eye view images overlap. Then, based on position of each pixel forming the difference region, it is evaluated which of the front and side cameras captures the solid object better, and an image of the common region in the synthetic bird's-eye view image is formed based on the image obtained from one of the cameras.