PURPOSE: To facilitate modification or reconstruction to higher grade equipments by a method wherein the differential frequency between the basic equipments and the higher grade equipments whose radiation frequency is high is superposed on the frequency of the basic composition at the transmitter side and the detection is performed in two steps at the receiver side.

CONSTITUTION: The 1st radiation frequency signal 26 from the 1st transmitter 24 and the 2nd radiation frequency signal 29 from the 2nd transmitter 27 are put into a frequency synthesizer 12 and converted into the 3rd radiation frequency signal 31, which is applied to a radiation coil in a probe 5. NMR signal 35 from the specimen is detected by the 1st amplifier-detector 14 with the 1st detection frequency signal 25 as a reference signal and converted into the 1st intermediate frequency signal 15. The 1st intermediate frequency signal is further detected by the 2nd amplifier-detector 16 with the 2nd detection frequency signal 28 as a reference signal. The 2nd intermediate frequency signal 17 is likewise subjected to the phase-detection by an intermediate frequency amplifier (detector) 18. Each circuit component is pulse-driven by a receiving gate pulse RG and irradiation gate pulse IG which have the phases nearly opposite to each other.