On one pair of opposed side faces (1a, 1b) of electro-optic crystal (1), grooves (3a, 3b) are formed so as to make bottom faces of the grooves approach each other and make a distance between the bottom faces shorter than a predetermined distance, and the pair of electrodes (5a, 5b) are formed in the grooves (3a, 3b) so as to fill the grooves nearly completely.