PURPOSE: To remarkably improve the machining accuracy of a workpiece at the time of grinding the workpiece of a magnetic material used for a magnetic head, etc., by holding the workpiece attaching face in a highly accurate flatness and determining the grinding quantity of the workpiece with the face as a reference face.

CONSTITUTION: An end face 2a which is machined in high accuracy is formed on the end part of a work spindle 2 rotated by a motor to transmit a turning force to a workpiece attaching board 1 while to hold the workpiece attaching board 1 in a closely attached condition. A true circular bottomed hole 1d having a taper part 1c is formed on the center on the other face 1b of the workpiece attaching board 1. And, around the center part on both faces of the workpiece attaching board 1 is rotatably held being interposed by the pointed part 3a on the end of a center 3 and the end face 2a of the work spindle 2, to rotate the workpiece attaching board 1 on which a workpiece W is attached. A cup type diamond grinding wheel B which rotate on its axis and which can be advanced/retracted grinds the grinding face of the workpiece W.