PURPOSE: To easily correct an unbalance quantity and to enhance the accuracy of balance correction by setting the centroid positions at the section of wire-shaped members for balance adjustment at a specified distance from a revolving shaft at all times.

CONSTITUTION: The central position of the wire shaped member 26 for balance adjustment fitted into a groove 25 for balance adjustment, i.e., a position 26a parting upward by a radius D1/2 from the bottom of the groove 25 for balance adjustment, is formed as a balance correcting surface 27. This balance correcting surface 27 is so set as to exist at a distance 1 from the centroid position 31 of the rotating body 50 consisting of a revolving sleeve 11 and a rotary polygon mirror 13, etc. The central position of the wire shaped member 29 for balance adjustment fitted into a groove 28 for balance adjustment, i.e., a position parting downward by a radius D1/2 from the bottom of the groove 28 for balance adjustment, is formed as a balance correcting surface 30. This balance correcting surface 30 is so set as to exist at a distance l2 from the centroid position 31 of the rotating body 50 consisting of the revolving sleeve 11 and the rotary polygon mirror 13, etc.