An electric field sensor which measures an electric field generated by a target utilizing an electro-optic effect, the electric field sensor including a light source, an electro-optic crystal on which light in a predetermined polarization state emitted from the light source is incident and which is subjected to the electric field generated by the target, a reference electric field applicator configured to apply an electric field based on a reference signal with a known signal level to the electro-optic crystal, a light receiver configured to receive light emitted from the electro-optic crystal and to convert the received light into an electric signal, and a separation corrector configured to separate the electric signal into a measurement signal based on the electric field generated by the target and the reference signal and to correct a signal level of the measurement signal on the basis of the signal level of the separated reference signal.