PURPOSE: To obtain an optical horizontal plane tilting device which determines the set height of a light receiving sensor visually by constituting the device so that a visible light and an invisible light are irradiated with the same optical axis.

CONSTITUTION: The light irradiated from an invisible light emitting element 11 becomes a parallel light by a lens 12, and the visible light irradiated from a visible light emitting element 8 becomes a parallel light by a lens 7, and they pass through a semitransparent prism 6 and are focused in a position of a focal length f2 by a correcting lens 5. The focused invisible light becomes a parallel light by an object lens 4 and is emitted in the horizontal direction by a double- reflection optical path changing prism 2, which is rotated with a certain rotation number, to form a laser horizontal plane of the invisible light. The angle of inclination of the horizontal plane is graduated to correspond to a scale 16 provided on a micro knob 15, and the laser plane and a visible light plane are inclined simultaneously.