PURPOSE: To enhance positioning workability, by moving a magnetic field measuring element on the axes γ, θ, and of a polar coordinates system by one axis when the magnetic field element is positioned on a spherical surface.

CONSTITUTION: A main shaft 19 is fixed to the axial core of a cylindrical object 22 by a support 20 and a magnetic field measuring element 33 is moved in a γ-axis direction and fixed to determine the position thereof on a γ-axis. Next, by moving a Z-axis body 25 in a Z-axis direction to fix the same, the position of the measuring element 33 on a Z-axis is determined and, by rotating a -axis handle 27, the position of the measuring element 33 on a -axis is determined. Further, by rotating a θ-axis handle 23, the Z-axis body 25 and a parallel link 21 are rotated around their axes through a key 34 and the position of the measuring element 33 on a θ-axis is determined. As mentioned above, by performing the positioning of the measuring element 33 on a spherical surface in succession by the main axis operation of one axis among the axes γ, θ, and using a polar coordinates system, the positioning and fixing of the measuring element 33 become easy.