Abstract: An electrooptic effect element of this invention has first, second, and third electrodes. The first and second electrodes are formed on a substrate having an electrooptic effect, in order to form a high-frequency electric field for polarizing a light beam, thus constituting a microstrip line, as in the conventional element. The third electrode is spaced apart from the second electrode on the substrate in order to form another electric field for canceling the polarized light by the high-frequency electric field. In an electrical signal waveform measuring apparatus of this invention, an electrical signal obtained from an optical detection system coupled to the electrooptic effect element is fed back to form a closed loop in the third electrode.