Provided is a surveying instrument including a light emitting section for emitting distance measuring light having an intensity distribution in which an intensity is high at the center and it became lower as approaching the periphery, a light receiving section for receiving the distance measuring light via a measurement object; and a distance-measurement optical system for guiding the light to the light receiving section via the measurement object. The surveying instrument performs a prism-mode and non-prism-mode measurements. The optical system includes an aperture diaphragm arranged in a distance-measurement optical path during the prism-mode-measurement. An opening of the aperture diaphragm has widths of greater than 0 and not greater than 0.5 times the full width at half maximum of the intensity distribution in a cross-section of non-prism-distance-measurement beam in the horizontal and vertical direction.