PURPOSE: To downsize and lighten a balance correcting member installed on a main spindle, to correct an unbalance in the rotation in a simple structure, and to realize a high-speed rotation.

CONSTITUTION: Three cylinders 8 are assembled respectively in the radial directions at the equal positions of 60° of a disk 7 for balancing installed to a main spindle 4 integrally. Each of the three cylinders 8 has a built-in piston 11 to be a balance weight in the slidable condition. A pressure fluid is fed in to a feeding passage 20 at the main spindle 4 side from a main shaft head 1 side through a minute clearance, and the three pistons 11 are moved in the radius directions of the disk 7 so as to eliminate an unbalance. Consequently, a moving balance is maintained automatically from the relative positionings of them.