1. An information processor comprising: a similarity data generation portion adapted to treat, as a reference block, each of a plurality of areas obtained by dividing one of left and right shot images shot with stereo cameras that shoot same space from left and right viewpoints, set up a search range in an other image for each of the reference blocks, and generate similarity data that represents the calculated similarity to the image in the reference block in association with a position within the search range; a result evaluation portion adapted to detect a position with a maximum similarity value for each piece of the similarity data and screen the detection result 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 result validated as a result of screening, calculate a position of a subject in a depth direction on a basis of the parallax, and generate a depth image by associating the position of the subject in the depth direction with an image plane; 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.