To determine and hold an operating point within a short time for performing highly reliable electric field measurement, as to an electric field sensor device.

A periodic signal is selectively impressed on an electrode 34 of a Mach-Zehnder interferometer optical modulator 30. An optical signal intensity-modulated by the periodic signal is detected as a feedback signal to carry out feedback control. A holding operation of light intensity at a maximum value is executed as to the extinction characteristic of the optical modulator 30. Similarly, a periodic signal is selectively impressed on the electrode 34 to execute a holding operation of light intensity at a minimum value as to the extinction characteristic of the optical modulator 30. A value proportional to one half of the sum of the maximum and minimum values is taken as reference electric power, making it possible to set and hold an operating point excelling in linearity within a short time.