To provide a magnetoencephalograph and a brain magnetic field measuring method which enable measurement with high accuracy without using a magnetic shield room.SOLUTION: A magnetoencephalograph M1 includes: a plurality of optical excitation magnetic sensors 1A for measuring a brain magnetic field; a plurality of magnetic sensors 2 for earth magnetism magnetic field correction for measuring a magnetic field related to an earth magnetism at each position of the plurality of optical excitation magnetic sensors 1A; a plurality of magnetic sensors 3 for active shield for measuring variable magnetic fields at each position of the plurality of optical excitation magnetic sensors 1A; an earth magnetism magnetic field correction coil; an active shield coil 9; a control device 5 for determining a current for generating a magnetic field for cancelling the magnetic field related to the earth magnetism on the basis of the measurement value of the plurality of magnetic sensors 2 for earth magnetism magnetic field correction, determining a current for generating the magnetic field cancelling the variable magnetic field on the basis of the measurement value of the plurality of magnetic sensors 3 for active shield, and outputting a control signal according to the determined current; and a coil power source 6 for outputting the current to each of the coils according to the control signal.SELECTED DRAWING: Figure 1