1. An object detecting system comprising: stereo-image taking means including a pair of image taking means mounted inside a front glass, the stereo-image taking means simultaneously taking images of an object around a vehicle in which the object detecting system is mounted, and outputting the images as a reference image and a comparative image; stereo matching means for performing, in the comparative image, stereo matching of a reference pixel block set in the reference image and having a predetermined number of pixels, and for calculating a parallax between the reference pixel block and a comparative pixel block specified in the comparative image, the parallax being calculated for each reference pixel block; object detecting means for calculating a distance of the object to the vehicle on the basis of the parallax and for detecting the object in the reference image on the basis of the distance; estimated-region setting means for setting, in the reference image and the comparative image, estimated regions where an image of the object is expected to be taken in a current frame, on the basis of the distance of the object in the reference image in the previous frame and a speed of the object relative to the vehicle calculated from the distance of the object to the vehicle; and determination means for determining whether an absolute value of a difference between an average luminance of the estimated region in the reference image and an average luminance of the corresponding estimated region in the comparative image is more than or equal to a predetermined threshold value, and for correlating information about the object detected in the estimated region of the reference image or information that the object is not detected, with information that noise is included in one of the taken images.