PURPOSE: To measure a magnetic field of a light emission point where effects of a leak magnetic field, an eddy current of a vacuum chamber, an operation mode, etc., are strong directly at a high precision by using an SOR light for measuring.

CONSTITUTION: When an SOR light 29 is radiated on a cathode 32, photo- electrons 48 are emitted from the cathode 32 by a photoelectric effect. These photo-electrons 48 are filtered at a grid 34, and only ones having a larger energy pass the grid 34 to be absorbed by an anode 36. An anode current Ia is thus sent to be measured by an ammeter 42. A computer 46 lets a grid voltage EG sweep, and measures a change of the anode current Ia at the time. A change pattern of the anode current Ia to the grid voltage EG is decided by an accumulated electron beam energy and a magnetic field at a light emission point. The magnetic field at the light emission point can thus be determined by the change pattern of the measured anode current Ia.