PURPOSE: To take a measurement accurately in safety by detecting the rotational position of a rotating shaft at every specific angle within one turn, and integrating the current attractive force double and obtaining the initial displacement position of the rotating shaft.

CONSTITUTION: The rotating shaft 1 is supported by an electromagnet 2 at a constant gap to a gap sensor 3. The rotational position of the shaft 1 is detected by a rotational position detector 4 and a rotational position detecting element 4', and a reset pulse detector 5' and a reset pulse detecting element 5'. When the shaft 1 is supported on the basis of the data of a reference gap setter 12, a control parameter rewrite command 17 is inputted. Then, the detectors 4 and 5 detect the specific angle of the shaft 1 within one turn, and integrate the attractive force at the angle double to store new data on the intial displacement position of the shaft 1 in a memory 8. Consequently, a measurement is taken without interrupting the control over a bearing.