To provide a workpiece machining device capable of improving machining quality and machining efficiency by suppressing the vibration of a rotary main spindle when a workpiece is machined.

The workpiece machining device machines the surface of the workpiece by relatively moving a machining tool and the workpiece 10. The device is provided with the rotatable rotary main spindle 3, a rotary table 9 to which the workpiece 10 is mounted and which is mounted to the end face 3a of the rotary main spindle 3 in a state that the rotation center 9a is deviated by a prescribed amount to the rotational axis 3b of the rotary main spindle 3, and a mounting means 8 for mounting a balance weight to the rotary main spindle 3.