1. A mobile object periphery monitoring apparatus comprising: an image capturing unit for capturing a stereo image including a base image and a reference image with a plurality of cameras; a plane area extracting unit for extracting a plane area in the stereo image, using the stereo image from said image capturing unit; and an object detecting unit for detecting an object in an area other than said plane area extracted by said plane area extracting unit, wherein the plane area extracting unit is configured to calculate a road plane area, extract image data from the stereo image corresponding to the road plane area to create a corrected image, and wherein the object detecting unit detects the object in the corrected image.