PURPOSE: To efficiently work upon both sides of machined object at the same time by composing an air chuck which is movable in the axial direction with more than one pair of pad rotary axles.

CONSTITUTION: A magnetic disc 3 is held by an air chuck 7 which is equipped movably in the axial direction to a rotary axle 6. With this condition, a motor 18 is driven, the rotary axle 6 is rotated, the magnetic disc 3 is rotated, at the same time, a pair of pad rotary axles 12, 13 are rotated, pads of these rotary axles 12, 13 are thrusted by air cushion to the magnetic disc, further, the pads are followed on the surface of the magnetic disc 3, the both surfaces of the magnetic disc 3 are polishing machined at the same time by said pads. This facilitates to offer double possibility of machining the both surfaces of the machined object and of improving efficiency and precision.