An optical device, comprising: a body; a telescope having an object lens, a transparent plate, and an eyepiece lens arranged in this order along an optical axis, the telescope being supported by the body for rotation about a horizontal axis (X-axis) orthogonal to the optical axis, the transparent plate supporting a reference scale drawn thereon, the reference scale being made of full or substantially full circles or circular arcs arranged concentrically and symmetrically about the optical axis, the transparent plate being arranged such that when the optical axis is collimated at a surface center (Ps) of a cylindrical body so that left and right edges (102L, 102R) of the cylindrical body appear on the transparent plate at an equal first distance from the optical axis an operator can read a scale number n α associated with the first distance using the reference scale; a measurement section for measuring a second distance from a reference point (P 0) where the optical axis crosses the horizontal axis (X-axis) to the surface center (Ps) and an elevation angle (θ) between a horizontal plane and the optical axis when the optical axis is collimated at the surface center (Ps); and an input section for allowing the operator to input the scale number (n α); characterized in that the optical device further comprises: a memory section for memorizing a relationship between the scale numbers n α and a view angle (α); and a calculation section for calculating the view angle (α) from the scale number (n α) input from the input section and for calculating coordinates of an internal center (Pc) of the cylindrical body located on the optical axis collimated at the surface center (Ps), from the second distance, the view angle (α), and the elevation angle (θ). a body; a telescope having an object lens, a transparent plate, and an eyepiece lens arranged in this order along an optical axis, the telescope being supported by the body for rotation about a horizontal axis (X-axis) orthogonal to the optical axis, the transparent plate supporting a reference scale drawn thereon, the reference scale being made of full or substantially full circles or circular arcs arranged concentrically and symmetrically about the optical axis, the transparent plate being arranged such that when the optical axis is collimated at a surface center (Ps) of a cylindrical body so that left and right edges (102L, 102R) of the cylindrical body appear on the transparent plate at an equal first distance from the optical axis an operator can read a scale number n α associated with the first distance using the reference scale; a measurement section for measuring a second distance from a reference point (P 0) where the optical axis crosses the horizontal axis (X-axis) to the surface center (Ps) and an elevation angle (θ) between a horizontal plane and the optical axis when the optical axis is collimated at the surface center (Ps); and an input section for allowing the operator to input the scale number (n α); characterized in that the optical device further comprises: a memory section for memorizing a relationship between the scale numbers n α and a view angle (α); and a calculation section for calculating the view angle (α) from the scale number (n α) input from the input section and for calculating coordinates of an internal center (Pc) of the cylindrical body located on the optical axis collimated at the surface center (Ps), from the second distance, the view angle (α), and the elevation angle (θ).