1. A monitoring system for monitoring surroundings of a vehicle by displaying images taken by a plurality of cameras provided at the vehicle on a display apparatus provided in a vehicle compartment, comprising: an image processing portion including: a boundary-determining means for determining a boundary of the neighboring images taken by the plurality of cameras for synthesizing the neighboring images in a horizontal direction by approximating the boundary to an exponential curve; a horizontally normalizing means for normalizing the neighboring images in a horizontal direction so that the boundary approximated to the exponential curve is converted into a linear vertical boundary; a vertically compensating means for compensating a vertical displacement between the neighboring images on the linear vertical boundary; and a pixel-adjusting means for adjusting the number of pixels of the synthesized image to the number of pixels suitable for displaying the synthesized image produced by the image processing portion on the display apparatus.