PURPOSE: To stabilize the electrostatic magnetic field by exciting at least two probes arranged in the electrostatic magnetic field by a frequency sweep system to control a magnet power source so that the time phase difference of an NMR signal from each probe becomes zero.

CONSTITUTION: NMR probes 3a and 3b are arranged at two points P and P' whose symmetrical center corresponds to the origin of a spherical space in an electrostatic magnetic field generated with a coil 1 connected to an electrostatic magnetic field power source 2. The probes 3a and 3b are excited in a high frequency with high frequency generators 4a and 4b, outputs of which are inputted into detection/low frequency amplifiers 7a and 7b. Here, NMR absorbed waveforms are detected and the phase difference therebetween is detected with a comparator circuit 9 through differentiation/shaping circuits 8a and 8b. Then, the power source 2 is so controlled as to zero the phase difference and thus, the electrostatic magnetic field can be stabilized.