PURPOSE: To eliminate the measuring error due to the temp. dependence of the change of the refractive index of a non-linear optical crystal or the deterioration of an electrostatic latent image by allowing linear polarized laser beam to be incident on the measuring head wherein the non-linear optical crystal is held between a measuring electrode and a rear electrode.

CONSTITUTION: A measuring head 14 is constituted by holding a non-linear optical crystal 26 between the measuring electrode 30 opposed to an object A to be inspected and a rear electrode 32. An electrostatic shield plate 34 having a fine hole 36 formed thereto is arranged to the rear surface of the measuring electrode 30 and the non-linear optical crystal 26 has Pockels effect changing the refractive index of said crystal corresponding to an electric field. Therefore, the surface potential of the object A to be inspected can be measured by forming the electric field corresponding to the surface potential of the object A to be inspected to the part corresponding to the fine hole 36 provided to the rear surface of the measuring electrode 30 of the non-linear optical crystal 26 in such a state that the measuring electrode 30 is opposed to the object A to be measured to change the refractive index of the non-linear optical crystal 26 and passing the measuring beam L of linear polarized beam having predetermined polarization to measure the polarization of the measuring beam L passing through the non-linear optical crystal 26.