PURPOSE: To correct irregularity of a position detection due to a variation of a magnetic field, by measuring an NMR (nuclear magnetic resonance) signal by adding only a uniform magnetic field, and also using the NMR signal obtained by said measurement, as a reference signal.

CONSTITUTION: A uniform magnetic field path 3 generates a uniform magnetic field M2 whose magnitude is a constant value, irrespective of a position in a measuring space S. An exciting signal circuit 4 generates a high frequency pulse of Larmor frequency, as an exciting signal, and obtains an NMR signal. Frequency of the NMR signal is stored in a reference frequency memory 10. Subsequently, a gradient magnetic field circuit 2 is made to generate a gradient magnetic field M1 which becomes "0" at a reference position P1 of the measuring space S, also reverses its direction before and behind said position, and varies its magnitude linearly. Subsequently, it is added to the uniform magnetic field M2, and a composite gradient magnetic field M3 is applied to the measuring space S.