The instrument for measuring lengths comprises a carrier, a rule secured to that carrier and provided with a scale, and an optoelectronic scanning unit comprising a scanning plate, light emitters and light receivers. The scanning unit is movable along the scale by means of an actuating member, which constitutes a feeler. Auxiliary tracks are provided for guiding the scanning unit along the scale. A part of the scanning unit is connected to the actuating member by means which are yieldable transversely to the longitudinal direction of the scale. Auxiliary tracks are provided for guiding a part of the scanning unit along the scale. The overall structure is simplified and reduced in overall size and a change of the clearance between the scanning plate and the rule is prevented in that only the scanning plate is guided in engagement with the auxiliary tracks, one of which is constituted by the rule. The light emitters and the light receivers of the scanning unit are secured to the actuating member, which is connected to the scanning plate by means which are yieldable transversely to the longitudinal direction of the scale.