To obtain a highly accurate automatic inclination correcting unit for surveying instrument in which a transparent liquid can be selected freely among currently available liquids while taking account of the viscosity.

The automatic inclination correcting unit for surveying instrument comprises a laser diode 1, a transparent case 2 containing a silicon oil C having a free liquid level C', a convex lens 3 for condensing the laser light L emitted from the diode 1 and passing the condensed laser light through the liquid level C' thus forming an image L1 of light source, and a convex lens 4 for condensing the laser light from the image L1 of light source and altering the inclination thereof. The image L1 of light source is then matched with the local length f2 of convex lens 4 and the distance (y) from the liquid level C' to the light source L1, the focal length 12, and the refractive index (n) of silicon oil C are set such that the laser light is emitted in the vertical direction from the convex lens 4. The refractive index (n) is determined when the silicon oil C is selected while taking account of the required viscosity and the (y) and f2 are set depending on the refractive index thus determined.