PURPOSE: To process a work easily and precisely into a desired form by providing a reciprocatory motion means for controlling the magnetic force of an electromagnet of a magnetic shaft bearing spindle electrically and reciprocating a rotary spindle in a processing cut direction based on a control signal.

CONSTITUTION: A synchronization detector 55 detects the position of a hole 21b of a work 21, and its detection timing is inputted to a CPU 50. This CPU 50 reads a preliminarily memorized return quantity based on the timing, and outputs a signal corresponding to a return quantity based on the value of a cut quantity following the advance of a cutting cycle. The CPU 50 operates a magnetic shaft bearing controller 47 for controlling the magnetic force of constant magnets 15, 16 of a magnetic shaft bearing spindle 11 electrically to make a rotary spindle 13 return toward the center of the work 21 for a predetermined quantity when the hole 21b of the work 21 gets to a grindstone 41. By this reciprocating the rotary spindle 13 of the magnetic shaft bearing spindle 11 in the processing cut direction, cutting work can be performed at a good roundness for an inner circumferential surface 21a of the hole 21b.