A potential measurement device including an irradiator for irradiating a sample with an electron beam; an extracting electrode disposed above the sample and extracting secondary electrons emitted from the part irradiated with the electron beam; a hemi-spherical grid analyzing the energy of the extracted secondary electrons; a lens disposed between the hemispherical grid and the sample and focusing both the electron beam and the secondary electrons; and a deflector disposed on the grid side with respect to the lens and producing electric and magnetic fields crossing perpendicularly to each other, which don't deflect the electron beam, but deflect only the secondary electrons.