A displacement measuring instrument wherein the instrument comprises a main scale and a carriage movable along the main scale having thereon mounted an index scale whose graduated surface is opposed through a minute space to a graduated surface of said main scale, and a value of relative displacement between the main scale and the carriage is measured from a variation in volume of a light emitted to the main scale and the index scale and transmitted therethrough or reflected thereat due to the relative movement between the scales, is of such an arrangement that at least part of travel guide mechanisms for said carriage comprises a groove formed in the moving direction of the carriage on either the graduated surface of said main scale or the outer surface of the carriage opposed to said graduated surface and convex guide means solidly secured to the rest of the abovementioned surfaces and guided in travel by the groove. With the abovedescribed arrangement, a stabilized travel guide performance can be obtained only by use of the graduated surface of the main scale without using the side face of the main scale as the scanning reference surface of the carriage.