PURPOSE: To shorten a necessary signal processing time, by taking the difference between a memory for accumulating data relating to a nuclear magnetic resonance signal part containing periodic noise and an increased memory for accumulating data relating only to periodic noise.

CONSTITUTION: An exciting RF pulse is applied to a transmitting-receiving coil 4 under the control of a calculator 7 and the generated NMR signal is detected by 0°- and 90°- phase detectors 10A, 10B to be accumulated in memories 14A, 14B so as to receive processing. At this time, the sampling data relating to the signal containing periodic noise of a power source hum are accumulated in the memories 14A, 14B while sampling data relating to the signal part only of the periodic noise are accumulated in increased memories 16A, 16B. Then, by taking the difference between both data by the calculator 7, periodic noise is removed. Therefore, the addition of a change-over device becomes unnecessary and the time for obtaining a desired signal can be shortened.