A target instrument comprising: a pole which has a ferrule part at a lower end part thereof, a prism which is provided on said pole and provided at a known distance from a lower end of said pole, and a terminal device provided on said pole, wherein said terminal device comprises an image pickup module which is fixed with a known attitude with respect to said pole, a tilt sensor which detects tilts in two axial directions which are orthogonal to each other and one of which axial directions is parallel to an optical axis of said image pickup module, and an arithmetic control module, wherein said image pickup module has an image pickup element and is configured to acquire an image which includes a reference object as a reference provided at a predetermined position in a measuring direction, wherein said optical axis of said image pickup module is orthogonal to an axis of said pole and orthogonally passes through said image pickup element, said image pickup element has a coordinate system which has an origin point at which said optical axis passes through the image pickup element, a positionof each pixel in said image pickup element is identified by said coordinate system, a light receiving signal from each pixel includes the positional information in the image pickup element, one of the coordinate axes of said coordinate system is orthogonal or parallel to an axis of said pole, said tilt sensor is configured to detect tilt angles in two axes orthogonal to each other of said target instrument with respect to the verticality, and said arithmetic control module is configured to calculate a tilt direction of said target instrument from a position of a figure of said reference object in said image, to calculate a tilt direction of said target instrument based on tilt angles in the two axial directions of said tilt sensor, to acquire a deviation between the tilt direction of said target instrument acquired from said image and the tilt direction acquired by calculating said tilt angles in the two axial directions based on an deviation amount caused by a rotation of the target instrument, and to correct the respective tilt angles in the two axial directions of said tilt sensor to tilt angles in directions parallel to said optical axis of said image pickup module and orthogonal to said optical axis based on said deviation in a state where the terminal device faces said reference object. wherein said image pickup module has an image pickup element and is configured to acquire an image which includes a reference object as a reference provided at a predetermined position in a measuring direction, wherein said optical axis of said image pickup module is orthogonal to an axis of said pole and orthogonally passes through said image pickup element, said image pickup element has a coordinate system which has an origin point at which said optical axis passes through the image pickup element, a positionof each pixel in said image pickup element is identified by said coordinate system, a light receiving signal from each pixel includes the positional information in the image pickup element, one of the coordinate axes of said coordinate system is orthogonal or parallel to an axis of said pole, said tilt sensor is configured to detect tilt angles in two axes orthogonal to each other of said target instrument with respect to the verticality, and said arithmetic control module is configured to calculate a tilt direction of said target instrument from a position of a figure of said reference object in said image, to calculate a tilt direction of said target instrument based on tilt angles in the two axial directions of said tilt sensor, to acquire a deviation between the tilt direction of said target instrument acquired from said image and the tilt direction acquired by calculating said tilt angles in the two axial directions based on an deviation amount caused by a rotation of the target instrument, and to correct the respective tilt angles in the two axial directions of said tilt sensor to tilt angles in directions parallel to said optical axis of said image pickup module and orthogonal to said optical axis based on said deviation in a state where the terminal device faces said reference object.