According to the present invention, when processing shafts in a centerless grinding machine, engagement between the shafts is prevented without rearranging the shafts, and thus outer circumferential faces of the shafts can be precisely processed. A shaft 12 is used in a hydrodynamic bearing device, and includes a cylindrical column formed with an outer circumferential face 12a forms a hydrodynamic radial bearing portion, a tapped hole 12j that is formed at a first end of the cylindrical column, and a first projecting portion 12b formed at a second end of the cylindrical column. When the first end and the second end of the shafts 12 are arranged side by side in an axis direction, the first projecting portion 12b of the shaft 12 can not interfere with the tapped hole 12j of the other shaft 12.