An information processor for detecting a position of a measurement target in a three-dimensional space using shot images, the information processor comprising: an image acquisition section adapted to acquire stereo image data obtained by capturing the measurement target in parallel with first and second cameras that are arranged at a given distance from each other; a detection plane definition portion adapted to define a detection plane in the three-dimensional space and establish a detection area obtained by projecting the detection plane onto an image shot by the first camera, i.e., one of the stereo images; a parallax correction area derivation portion adapted to derive a parallax correction area in an image shot by the second camera, i.e., one of the stereo images, by moving the same area as the detection area in a direction of eliminating parallax by as much as a magnitude of parallax determined by a position of the detection plane in a depth direction; a matching portion adapted to perform matching between the image of the detection area of the image shot by the first camera and the image of the parallax correction area of the image shot by the second camera; a calibration portion adapted to acquire the position of the measurement target in the depth direction by acquiring an actual size of the measurement target from a database and comparing an apparent size in the shot image and the actual size, define a detection plane at that position, calibrate the information processor, perform matching by changing how much the parallax correction area is moved, and correct a relationship between the position in the depth direction and how much the parallax correction area is moved by extracting how much the parallax correction area is moved which provides proper matching rating; and a detection result output section adapted to output a result of matching performed by the matching portion. an image acquisition section adapted to acquire stereo image data obtained by capturing the measurement target in parallel with first and second cameras that are arranged at a given distance from each other; a detection plane definition portion adapted to define a detection plane in the three-dimensional space and establish a detection area obtained by projecting the detection plane onto an image shot by the first camera, i.e., one of the stereo images; a parallax correction area derivation portion adapted to derive a parallax correction area in an image shot by the second camera, i.e., one of the stereo images, by moving the same area as the detection area in a direction of eliminating parallax by as much as a magnitude of parallax determined by a position of the detection plane in a depth direction; a matching portion adapted to perform matching between the image of the detection area of the image shot by the first camera and the image of the parallax correction area of the image shot by the second camera; a calibration portion adapted to acquire the position of the measurement target in the depth direction by acquiring an actual size of the measurement target from a database and comparing an apparent size in the shot image and the actual size, define a detection plane at that position, calibrate the information processor, perform matching by changing how much the parallax correction area is moved, and correct a relationship between the position in the depth direction and how much the parallax correction area is moved by extracting how much the parallax correction area is moved which provides proper matching rating; and a detection result output section adapted to output a result of matching performed by the matching portion.