A surveying device comprising: a rotating unit including an optical device that performs optical surveying, the rotating unit being horizontally rotatable; a horizontal rotation angle sensor that measures a horizontal rotation angle of the rotating unit; a tilt sensor; arranged to the rotating unit, the tilt sensor being configured to measure a tilt of the rotating unit relative to the direction of gravity; a tilt calculating unit that calculates a tilt of the rotating unit after the rotating unit rotates, on the basis of outputs from the horizontal rotation angle sensor and the tilt sensor before the rotating unit rotates and on the basis of an output from the horizontal rotation angle sensor after the rotating unit rotates; and a support that supports the rotating unit so that the rotating unit is horizontally rotatable, characterized by the tilt sensor being configured to measure the tilt of the rotating unit relative to the direction of gravity by optically measuring a tilt of a liquid surface, the support being arranged with an additional tilt sensor that measures a tilt of the support, the additional tilt sensor being configured to measure the tilt of the support while the rotating unit rotates horizontally, the tilt that is calculated by the tilt calculating unit being represented by an X component value and a Y component value (x, y) under the conditions that a vertical vector along a vertical axis of the rotating unit is projected on an X-Y horizontal plane, and the X component value and the Y component value (x, y) being corrected on the basis of a value measured by the additional tilt sensor. a rotating unit including an optical device that performs optical surveying, the rotating unit being horizontally rotatable; a horizontal rotation angle sensor that measures a horizontal rotation angle of the rotating unit; a tilt sensor; arranged to the rotating unit, the tilt sensor being configured to measure a tilt of the rotating unit relative to the direction of gravity; a tilt calculating unit that calculates a tilt of the rotating unit after the rotating unit rotates, on the basis of outputs from the horizontal rotation angle sensor and the tilt sensor before the rotating unit rotates and on the basis of an output from the horizontal rotation angle sensor after the rotating unit rotates; and a support that supports the rotating unit so that the rotating unit is horizontally rotatable, characterized by the tilt sensor being configured to measure the tilt of the rotating unit relative to the direction of gravity by optically measuring a tilt of a liquid surface, the support being arranged with an additional tilt sensor that measures a tilt of the support, the additional tilt sensor being configured to measure the tilt of the support while the rotating unit rotates horizontally, the tilt that is calculated by the tilt calculating unit being represented by an X component value and a Y component value (x, y) under the conditions that a vertical vector along a vertical axis of the rotating unit is projected on an X-Y horizontal plane, and the X component value and the Y component value (x, y) being corrected on the basis of a value measured by the additional tilt sensor. <EOS>