The invention provides a surveying instrument, which comprises a control device having a reference clock signal generating unit for producing a reference clock signal. The control device produces a time signal to indicate a standard time from a signal acquired from the GNSS device, puts a time stamp on the time signal, associates the time signal with the reference clock signal, measures a horizontal angle at a moment when the telescope unit sights the sun and acquires an image signal, puts a time stamp on the image signal, calculates a standard time at a time of image acquisition, calculates an azimuth angle of the sun at the standard time based on a latitude and a longitude of a point where the surveying instrument is installed and a standard time at the time of image acquisition, and updates a horizontal angle by the azimuth angle.