PURPOSE:To know simultaneously a deflection and a deflection angle of a front position of a shielding machine by providing successively the first and the second half mirrors vertically to an optical axis of a convex lens, by positioning them at the convex lens side from its focus, in front of the convex lens. CONSTITUTION:When a focusing light C for projecting a tunnel plan center line is inclined by an angle alpha against a center shaft 3 of a shielding machine, the light C passes through the second half mirror 9, a part of said light is reflected by the first half mirror 10, and travels to the direction of the second half mirror 9, and on the other hand, the remaining light passes through. Also, as for a light spot which passes first the first mirror 10 placed at a lens 6 side at a distance X from a focus 7 of a convex lens 6 of a focal distance (f) and is irradiated to a target 8, its position is read by a scale plate 11, and its light spot shows a deflection of a point to be measured A. Also, a light spot B' which is reflected once by the second mirror 9, reaches the first mirror 10 and is irradiated to the target 8 is equal to a light spot B which passes through by a distance X from the first mirror 10. The distance from the optical axis of this light spot B' is f.tanalpha which is the same as the spot B.
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