A surveying instrument comprising a monopod which is installed on a reference point, said monopod comprises a reference plate provided at said lower end of said monopod, a reference target having a cross and provided on an upper surface of said reference plate, and a ferrule provided on a lower surface of said reference plate in such a manner that an intersection of said cross coincides with an axis of said ferrule, a surveying instrument main body which has a machine center and a reference optical axis, and wherein a distance in horizontal direction and a distance in vertical direction between the lower end of the ferrule of said monopod and said machine center are known, two auxiliary legs which extend downward from a middle portion of said monopod at a predetermined angle and are foldably coupled with said monopod, and a lower image pickup module which is provided at a known position with respect to said surveying instrument main body and has an image pickup optical axis directed downward at the predetermined angle with respect to said reference optical axis, wherein said surveying instrument main body comprises a distance measuring unit configured to measure a distance to an object to be measured, a measuring direction image pickup module configured to acquire an observation image including said object to be measured in a predetermined relationship with said reference optical axis, and an arithmetic control module configured to make said distance measuring unit measure a predetermined measuring point, to make said measuring direction image pickup module acquire an image of said object to be measured, and to make said lower image pickup module acquire a lower image with reference to said image pickup optical axis, wherein said surveying instrument main body is supported by three points by said monopod and said auxiliary legs, and a positional change or a rotational change of said surveying instrument main body is calculated based on a change of said lower image, wherein said lower image is acquired in such a manner that said reference target and at least one reference object are included, and said arithmetic control module calculates a rotation angle of a longitudinal direction and a rotation angle of a lateral direction of said surveying instrument main body based on a displacement of a position of at least one of said reference target and said reference object in said lower image. said monopod comprises a reference plate provided at said lower end of said monopod, a reference target having a cross and provided on an upper surface of said reference plate, and a ferrule provided on a lower surface of said reference plate in such a manner that an intersection of said cross coincides with an axis of said ferrule, a surveying instrument main body which has a machine center and a reference optical axis, and wherein a distance in horizontal direction and a distance in vertical direction between the lower end of the ferrule of said monopod and said machine center are known, two auxiliary legs which extend downward from a middle portion of said monopod at a predetermined angle and are foldably coupled with said monopod, and a lower image pickup module which is provided at a known position with respect to said surveying instrument main body and has an image pickup optical axis directed downward at the predetermined angle with respect to said reference optical axis, wherein said surveying instrument main body comprises a distance measuring unit configured to measure a distance to an object to be measured, a measuring direction image pickup module configured to acquire an observation image including said object to be measured in a predetermined relationship with said reference optical axis, and an arithmetic control module configured to make said distance measuring unit measure a predetermined measuring point, to make said measuring direction image pickup module acquire an image of said object to be measured, and to make said lower image pickup module acquire a lower image with reference to said image pickup optical axis, wherein said surveying instrument main body is supported by three points by said monopod and said auxiliary legs, and a positional change or a rotational change of said surveying instrument main body is calculated based on a change of said lower image, wherein said lower image is acquired in such a manner that said reference target and at least one reference object are included, and said arithmetic control module calculates a rotation angle of a longitudinal direction and a rotation angle of a lateral direction of said surveying instrument main body based on a displacement of a position of at least one of said reference target and said reference object in said lower image.