Rotating angle measuring device and method for rotary object

1) Field of the Invention A rotating angle measuring device for a rotary object includes a holder mounted detachably to the object so that the holder is rotated together with the object; a rotating angle sensor coupled to the holder and having a shaft rotated relative to the holder; a level coupled to the shaft and having an initial value as a datum; and an operation part which rotates the shaft. A rotating angle of the shaft is measured by rotating the shaft using the operation part until the changed value of the level due to the rotation of the object is returned to the initial value, thereby determining a rotating angle of the object. A rotating angle measuring method includes the steps of mounting a rotating angle sensor and a level to the object, the level being coupled to a shaft of the sensor and rotated with the shaft; setting an initial value of the level; rotating the object at a target rotating angle; returning the changed value of the level due to the rotating step to the initial value by rotating the level; and measuring a practical rotating angle of the object by measuring with the sensor the rotating angle of the shaft during the returning step. The method further comprises a compensating step after the measuring step, in which the practical rotating angle of the object is analyzed and a deviation between the target rotating angle and the practical rotating angle is calculated and compensated for.