1. An information processor comprising: an image acquisition section enabled to receive image data from an imaging device equipped with stereo cameras for shooting the same space from left and right viewpoints; a position information generation section adapted to generate subject position information on the basis of images of a subject shot by the stereo-camera imaging device, the position information generation section including: a similarity data generation portion adapted to treat, as a reference block, each of a plurality of reference-image areas obtained by dividing one of either left or right shot images shot with the imaging-device stereo cameras, to set up, for each of the reference blocks in the one of the either left or right shot images, a search-range image area in the other of the either left or right shot images, and to generate similarity data representing by-position similarity of the search-range image area to the reference-image area, calculated in association with position within the search range; a result evaluation portion adapted to detect in the similarity data on a per-piece basis a position with a maximum similarity value and screen the detection result as either validated or invalidated, by making a given evaluation of the similarity; a depth image generation portion adapted to find a parallax for each of the reference blocks using the detection results validated as a result of the screening by the result evaluation portion, calculate a position of a subject on a basis of the parallax, and generate a depth image that represents the position of the subject in the depth direction as pixel values; and an output information generation section adapted to perform given information processing on a basis of the subject position in a three-dimensional space using the depth image and output the result of information processing.