With the progress in electronic equipment, evaluation of the electromagnetic compatibility (EMC) of the equipment has become an important issue. To evaluate the EMC it is necessary to accurately understand the electromagnetic field surrounding the equipment. Therefore, various sensors for measuring the electric field have been developed. Among them, since the electric field sensor using a Mach–Zehnder interferometer features wide bandwidth, high sensitivity, and high resolution, its application in EMC measurement is expected. However, its sensitivity changes at a frequency higher than a few megahertz due to unknown reasons and this makes the intensity measurement of the electric field much more difficult. In this paper, the sensitivity variation is considered to be caused by the resonance due to the piezoelectric effect of the optical crystal substrate used to form the optical modulator. By comparing the measured and theoretical resonance frequencies it is found that the sensitivity variation is due to the resonance in the width direction of the substrate. Based on this result, a method is proposed to suppress the resonance by varying the width of the substrate along the propagating direction. Finally, the improvement of the frequency response is demonstrated by using an optical interferometer with a variable width crystal substrate as the electric field sensor. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 83(11): 76–84, 2000