An optical modulator comprising: a slab type two-dimensional photonic crystal layer comprising a dielectric layer comprising an upper face and a lower face, the dielectric layer having a dielectric constant, a plurality of sets of lattice columns (7A-7G) each comprising dielectric pillars and a waveguide portion formed in said photonic crystal layer, a defect region where said dielectric pillars are not formed constituting said waveguide portion, the lattice columns being arranged in first, second, third and higher sets on the right and left sides of the waveguide portion, respectively at increasing first, second, third and higher distance orders from the waveguide portion; a ground electrode and a signal electrode formed on said upper face of said dielectric layer and for applying a modulating voltage on light propagating in said waveguide portion; a high dielectric layer laminated on said lower face of said dielectric layer, wherein the high dielectric layer and the dielectric layer are formed of different materials; and a low dielectric portion provided under said lower face of said dielectric layer and facing said waveguide portion and (i) the low dielectric portion faces said first, second and third sets of lattice columns (7A, 7B, 7C), the low dielectric portion not being present under fourth sets of lattice columns and higher, the high dielectric layer being provided under said fourth sets of lattice columns and higher, or (ii) the low dielectric portion faces said first, second, third and fourth sets of lattice columns (7A, 7B, 7C, 7D), the low dielectric portion not being present under fifth sets of lattice columns and higher, the high dielectric layer being provided under said fifth sets of lattice columns and higher, or (iii) the low dielectric portion faces said first, second, third, fourth and fifth sets of lattice columns (7A, 7B, 7C, 7D, 7E), the low dielectric portion not being present under sixth sets of lattice columns and higher, the high dielectric layer being provided under said sixth sets of lattice columns and higher, wherein said low dielectric portion has a dielectric constant lower than that of the high dielectric layer. a slab type two-dimensional photonic crystal layer comprising a dielectric layer comprising an upper face and a lower face, the dielectric layer having a dielectric constant, a plurality of sets of lattice columns (7A-7G) each comprising dielectric pillars and a waveguide portion formed in said photonic crystal layer, a defect region where said dielectric pillars are not formed constituting said waveguide portion, the lattice columns being arranged in first, second, third and higher sets on the right and left sides of the waveguide portion, respectively at increasing first, second, third and higher distance orders from the waveguide portion; a ground electrode and a signal electrode formed on said upper face of said dielectric layer and for applying a modulating voltage on light propagating in said waveguide portion; a high dielectric layer laminated on said lower face of said dielectric layer, wherein the high dielectric layer and the dielectric layer are formed of different materials; and a low dielectric portion provided under said lower face of said dielectric layer and facing said waveguide portion and (i) the low dielectric portion faces said first, second and third sets of lattice columns (7A, 7B, 7C), the low dielectric portion not being present under fourth sets of lattice columns and higher, the high dielectric layer being provided under said fourth sets of lattice columns and higher, or (ii) the low dielectric portion faces said first, second, third and fourth sets of lattice columns (7A, 7B, 7C, 7D), the low dielectric portion not being present under fifth sets of lattice columns and higher, the high dielectric layer being provided under said fifth sets of lattice columns and higher, or (iii) the low dielectric portion faces said first, second, third, fourth and fifth sets of lattice columns (7A, 7B, 7C, 7D, 7E), the low dielectric portion not being present under sixth sets of lattice columns and higher, the high dielectric layer being provided under said sixth sets of lattice columns and higher, (i) the low dielectric portion faces said first, second and third sets of lattice columns (7A, 7B, 7C), the low dielectric portion not being present under fourth sets of lattice columns and higher, the high dielectric layer being provided under said fourth sets of lattice columns and higher, or (ii) the low dielectric portion faces said first, second, third and fourth sets of lattice columns (7A, 7B, 7C, 7D), the low dielectric portion not being present under fifth sets of lattice columns and higher, the high dielectric layer being provided under said fifth sets of lattice columns and higher, or (iii) the low dielectric portion faces said first, second, third, fourth and fifth sets of lattice columns (7A, 7B, 7C, 7D, 7E), the low dielectric portion not being present under sixth sets of lattice columns and higher, the high dielectric layer being provided under said sixth sets of lattice columns and higher,