1. An object detecting device comprising: a pair of imaging means for taking an image of a three-dimensional object; disparity data computing means for computing disparity data for each region based on the images obtained by the pair of imaging means, each region being obtained by dividing the image; gray-scale image generating means for computing a gray-scale value indicating a distance to the imaging means based on the disparity data computed for each region and generating a gray-scale image having the gray-scale value corresponding to each region; model storage means in which a model modeling the three-dimensional object is stored; correlation value computing means for computing a correlation value indicating a similarity between the model and an image region in the gray-scale image, the model being a two-dimensional image having a geometric feature when the three-dimensional object is viewed from a direction in which the imaging means are located, each region obtained by dividing the two-dimensional image having a gray-scale value indicating a distance to the imaging means, of a corresponding portion of the three-dimensional object, the correlation value being computed based on gray-scale values of the model and gray-scale values of the image region in the gray-scale image; and object detecting means for detecting the three-dimensional object by detecting an image region in the gray-scale image, having a highest correlation value with the model.