PURPOSE: To allow optimum gain in a transmitting system to be automatically set up depending on photographing conditions, and thereby obtain an excellent image with a high S/N ratio at all time by providing both a preliminary measuring means and a transmitting system gain control means allowing an optimum gain in a transmitting system to be obtained depending on photographing conditions prior to the operation of the measurement.

CONSTITUTION: In a transmitting system 2, a modulating system 5 allows high frequency pulses outputted from a high frequency oscillator 6 to be modulated in amplitude by controlling a computer 4 so as to let the pulses be amplified by an amplifier 7 for letting them be fed to a transmitting high frequency coil 8 so that they are irradiated to a living body 1 to be examined as electromagnetic waves. In a receiving system 3, the high frequency pulses are irradiated to the living body 1 to be examined, NMR signals generated from the living body 1 to be examined are received by a receiving high frequency coil 9, they are amplified thereafter by an amplifier 10 so as to be inputted to a phase detector 11. After they have been detected, they are converted into digital signals by an A/D converter 12 so as to be inputted into the computer 4. The computer 4 performs operations for reconstructing an image. In a variable attenuator 14, attenuation is changed in order stepwise every each preliminary measurement made, optimum attenuation is finally set up when the NMR signal attains the maximum peak value of it.