A first and a second sensor array 21 and 22 arranged at the approximate image forming plane of a pair of optical systems, and a third sensor array 23 arranged at a spacing h from the first and second sensor arrays 21 and 22. The image interval X is corrected to a standard image interval K=X (1-;tan /tan(+)) when the object image intersects the optical base length R₀ via the object image inclination =tan^-;1 (h/z) calculated from the image forming positions of the object images T₁ and T₂ detected by the sensor arrays 21 and 22, and the dislocation angle formed by the sensor and the optical system detected after assembling the device.