1. A vehicle surrounding monitoring system comprising: an image-capturing portion that captures an image of at least a portion of an area surrounding a vehicle; a storage portion that stores the image captured by the image-capturing portion; a display-image generation portion that generates a synthesized image for display using a latest image that is captured by the image-capturing portion, and a previous image that is captured before the latest image is captured; and a display portion on which the synthesized image is displayed, wherein the display-image generation portion decreases a level of visibility of an image region relating to the previous image, according to an elapsed time after the previous image is captured.