To measure the unbalance of a rotor in an element stage such as after casting in a manufacturer.

An apparatus comprises a spindle 11 for supporting a rotary table 2 for rotation, a vibration detector 18 for detecting the amount vibration in a perpendicular direction of the shaft of the rotating spindle 11, an encoder 17 for detecting the rotation of the spindle 11, and a retaining tool 3 that is provided on the rotary table 2 and retains the rotor material 4 so that the rotary axis coincides with a rotary axis Ax of the spindle 11.