An optical modulator (3, 3a - 3h), comprising: a base part (31, 31a - 31c) which is a plate-like member formed of a material of which the refractive index is changed by an electric field, having a periodically-poled structure in which a first polarization part and a second polarization part which are opposite in the direction of polarization are alternately arranged in a polarization-part array direction; and a modulation part having a first electrode portion provided on one main surface of said base part (31, 31a - 31c) and a second electrode portion provided on the other main surface of said base part (31, 31a - 31c), which applies voltage between said first electrode portion and said second electrode portion to thereby cause a periodic change of said refractive index in said polarization-part array direction in said periodically-poled structure, wherein a linear light beam spreading out in said polarization-part array direction enters said base part (31, 31a - 31c) from one end surface thereof, being guided inside said base part (31, 31a - 31c) in a traveling direction in which said light beam is diffracted by said periodic change of said refractive index, and exits from the other end surface, said first polarization parts and said second polarization parts are each a strip-like part extending in a direction perpendicular to the polarization-part array direction, said first electrode portion comprises a plurality of first electrodes arranged in an electrode array direction parallel to said polarization-part array direction and perpendicular to said traveling direction, each of said plurality of first electrodes is disposed on a polarization part group including a predetermined number of first polarization parts and second polarization parts which are arranged successively in said polarization-part array direction, said plurality of first electrodes are individually connected to an electrical potential applying part, said optical modulator (3, 3a - 3b) being arranged for causing a periodic change in the refractive index in the polarization-part array direction in each of polarization pair groups corresponding to each of said plurality of first electrodes by applying a voltage between each of said plurality of first electrodes and said second electrode portion, and voltages applied between said plurality of first electrodes and said second electrode portion are equal and individually controlled, characterized in that, said second electrode portion comprises a single second electrode as a common electrode opposed to said plurality of first electrodes. a base part (31, 31a - 31c) which is a plate-like member formed of a material of which the refractive index is changed by an electric field, having a periodically-poled structure in which a first polarization part and a second polarization part which are opposite in the direction of polarization are alternately arranged in a polarization-part array direction; and a modulation part having a first electrode portion provided on one main surface of said base part (31, 31a - 31c) and a second electrode portion provided on the other main surface of said base part (31, 31a - 31c), which applies voltage between said first electrode portion and said second electrode portion to thereby cause a periodic change of said refractive index in said polarization-part array direction in said periodically-poled structure, wherein a linear light beam spreading out in said polarization-part array direction enters said base part (31, 31a - 31c) from one end surface thereof, being guided inside said base part (31, 31a - 31c) in a traveling direction in which said light beam is diffracted by said periodic change of said refractive index, and exits from the other end surface, said first polarization parts and said second polarization parts are each a strip-like part extending in a direction perpendicular to the polarization-part array direction, said first electrode portion comprises a plurality of first electrodes arranged in an electrode array direction parallel to said polarization-part array direction and perpendicular to said traveling direction, each of said plurality of first electrodes is disposed on a polarization part group including a predetermined number of first polarization parts and second polarization parts which are arranged successively in said polarization-part array direction, said plurality of first electrodes are individually connected to an electrical potential applying part, said optical modulator (3, 3a - 3b) being arranged for causing a periodic change in the refractive index in the polarization-part array direction in each of polarization pair groups corresponding to each of said plurality of first electrodes by applying a voltage between each of said plurality of first electrodes and said second electrode portion, and voltages applied between said plurality of first electrodes and said second electrode portion are equal and individually controlled, characterized in that, said second electrode portion comprises a single second electrode as a common electrode opposed to said plurality of first electrodes.