PURPOSE: To exactly detect the fluctuation quantity of high-frequency magnetic fields without generating noises, etc.

CONSTITUTION: The signal receiving sensor for MIR which detects the intensity of a nuclear magnetic resonance signal has a light source 1 which generates light possessing a prescribed plane of polarization, a magneto-optical material means 4 at least a part of which exists in the static magnetic field for the MRI and which has the property to rotate the plane of polarization at the time of transmission of the light from the light source according to the intensity of the ambient magnetic fields and detecting means 7, 8 which detect the degree of rotation of the plane of polarization of the light past the magneto-optical material means 4.