The invention concerns a device (10) for measuring the horizontal distance separating two horizontal parallel axis of a mechanical structure, comprising:- a spirit level (3) defining a direction Dh, said direction Dh being horizontal when the spirit level is level,- a transparent support (1) defining a plane P on one of its sides and bearing on the said side a plurality of parallel marking lines (2a-2f), said lines extending in a direction perpendicular to the direction Dh and defining a scale,- a positioning element (8) defining a direction Dt orthogonal to the plane P.
