PURPOSE: To prevent an error to be generated at both of a control time and an imaging time, by preliminarily inputting a predetermined dummy signal in place of a nuclear magnetic resonance (NMR) signal to perform the orthogonal detection and A/D conversion of said signal and measuring the characteristics of two phase systems A, B from the obtained value and applying compensation to the measuring data (real part data and false part data) of the systems A, B at the imaging time of an examinee.

CONSTITUTION: A change-over device 8 is newly provided to the input part of an MNR signal 01. A computer 7 inputs the MNR signal 01, a zero signal and a reference oscillation signal 1a to the common terminal 31 of phase detectors 3a, 3b through contacts 8a, 8b, 8c. The computer 7 changes over the change-over device 8 to the contact 8b before the imaging of an examinee to select the zero signal and calculates real part C offset po and false part DC offset qo as the DC offset values of systems A, B and continuously changes over the change-over device 8 to the contact 8c to input the reference oscillation signal 1a and calculates real part gain difference measuring data p1 and false part gain difference measuring data q1. By this method, the zero signal and the signal of predetermined phase difference from a reference oscillator 1 are applied and DC offset and gain difference are automatically compensated at the imaging time of the examinee.