A light control element is provided with a thin board having electro-optical effects; an optical waveguide formed on the thin board; and a control electrode for controlling light that passes through the optical waveguide. The light control element performs speed matching between a microwave signal applied to the control electrode and the light, impedance matching of the microwaves, reduction of a driving voltage and high speed operation. In the control electrode of the light control element, a signal electrode and a grounding electrode are arranged on an upper side of the thin board, and on a lower side of the thin board, a second electrode including the grounding electrode is arranged, through a low refractive index layer entirely formed in the length direction of the signal electrode, with a width wider than that of the signal electrode.