To precisely detect the center of a Hall element mounted on a probe by minutely moving the Hall element within magnetic fields differed in distribution to regularly measure the output according to the distribution of magnetic fields, and judging the arrangement position at the present time of the Hall element from the degree of change of the output.

Within a space surrounded by electromagnetic coils 3A, 3B, 3C, 3D, a distribution of electric fields is formed in lateral direction in x-y plane to each electromagnetic coil 3A-3D. A probe 4 is positioned vertically (z-axially) to the x-y plane. The probe 4 is moved in x-direction so as to cross the front surface of S-pole of the coil 3A, then turned to y-direction, and moved so as to cross the front surface of N-pole of the coil 3B. The output from the Hall element of the probe 4 is regularly detected to track the change of the output. The arrangement position of the Hall element at the present time to the probe 4 forming the support body can be judged from the degree of change of the output.