A surveying instrument comprises a monopod installed on a reference point, a surveying instrument main body provided at a known distance from a lower end of the monopod and at a known angle with respect to an axis of the monopod and having a reference optical axis, wherein the surveying instrument main body performs an image pickup of an object to be measured and scans the object to be measured by a closed loop scan pattern respectively at a pre-rotation and a post-rotation of the surveying instrument main body, obtains cross points of a locus of a scan pattern of the pre-rotation and the post-rotation, obtains deflection angles of the cross point with respect to the reference optical axis of the pre-rotation and the post-rotation, and calculates a rotation angle of the surveying instrument main body based on a deviation between both of the deflection angles.