1. A stereo image processor, comprising: a stereo image acquiring section including a first imaging system and a second imaging system, the stereo image acquiring section for acquiring a base image by taking an image of an object by the first imaging system and a comparative image by taking an image of the object by the second imaging system; an image matching section acquiring the base image and the comparative image from the stereo images, extracting a first partial image centered at a predetermined reference point from the base image with a first window function, and deriving a corresponding point from a predefined area in the comparative image, the corresponding point being a center point in a second partial image where a level of difference in image luminance is minimum relative to the first partial image; a filter section acquiring the base image and the comparative image from the stereo images, extracting a third partial image centered at a predetermined reference point from the base image with a second window function, extracting a fourth partial image centered at the corresponding point from the comparative image, and generating an output signal sequence based both on values of a sequence of luminance values of respective coordinate points included in the third partial image with an order thereof reversed with respect to coordinates and on luminance values of respective coordinate points included in the fourth partial image; and a peak position detecting section for detecting an output signal with a maximum output value from the acquired output signal sequence to determine disparity between the base image and the comparative image based on a position of the detected output signal in the output signal sequence.