PURPOSE: To make it possible to instruct a center-hole machining position, so that unbalance of a body under test lies within a specified range, by providing a circuit, which computes the amount and direction of deviation between the central axis of rotation of the body under test and the main inertial axis on both right and left end surfaces of the body under test.

CONSTITUTION: A body under test is supported by right and left bearings 1L and 1R and rotated by a rotation driving mechanism. The unbalance signals generated by the rotation are detected by detectors 2L and 2R, which are provided in the right and left bearings 1L and 1R. A mark and the like, which are attached to the body under test are detected by a photocell 3, and the rotation phase of the body under test is detected. The outputs of the detectors 2L and 2R and the output of the photocell 3 are guided to dynamic balance measuring circuit 4. After specified operation, unbalance vectors We and Wr on the right and left correcting surfaces (l) and (r), which are provided on the body under test, are computed. The vectors are indicated on a display device and introduced to a center-hole-position operating circuit 5.