To measure a leakage current form an electron beam on a real-time basis, since a change of a current value in an electron beam gives a big influence to the result of an exposure in an electron beam exposure device, and if there is a leakage current from the beam, the change of the current value possibly causes destruction of components and unexpected radiation of an X-ray, and an exposure device may be seriously damaged, to say nothing of stability of the electron beam.

In addition to the current value of the electron beam used for exposure, the value of the current leaked from the electron beam is measured, and is compared with a current value to be supplied on a real-time basis to grasp the leakage. As a method for implementing it, the leakage is measured in a power source to accelerate the electron beam discharged from a cathode.