To install a sensor detection surface in the same surface as a fixing nut or in a lower surface than the nut with reducing in cost and maintaining or elongating the detection distance on the basis of a device in shape or material of the fixing nut.

A first chamfered part (31) is formed on the inner circumference of an aperture (35) opened in the surface on the front end side of a metal base of the fixing nut (3) on the front side, while a second chamfered part (32) is formed on the inner circumference of an aperture (36) opened in the back end side of the metal base. The first chamfered part is formed as an outsized chamfered part for the purpose of making the path of leaked magnetic flux from the front end of the metal base blank, while the second chamfered part is formed as a regular size chamfered part for the purpose of burring necessary for machining process.