PROBLEM TO BE SOLVED: To provide a rail displacement amount measuring apparatus of high precision at a low cost which can reduce manhours necessary for angle adjustment of a projector and a light receiving apparatus.
SOLUTION: A projector 51 and a light receiving apparatus 52 are fixed to a substrate 21a of a fixing base 21 via fixing seats 41 and 42, respectively. The projection axis Ta of the projector 51 and the center axis of a cylindrical cabinet 61 are arranged on the same axis. The light receiving axis Ja of the light receiving apparatus 52 and the center axis of a cylindrical cabinet 71 are arranged on the same axis. The axes Ta, Ja are arranged on the same axis. A glass mirror 53 for projection is so positioned that the projection axis Tb has a specified angle θ3 in the direction rectangular to a rail 91, and fixed on the substrate 21a of the fixing base 21, via a retainer 43. A glass mirror 54 for light receiving is so positioned that the light receiving axis Jb has a specified angle θ1 to the direction rectangular to the rail 91, and fixed on the substrate 21a of the fixing base 21 of the fixing base 21, via a retainer 44.