An optical modulator device, comprising: a sheet-like substrate which has a top surface and a bottom surface, exhibits an electro-optical effect and has a thickness of 10 μm or less; an optical waveguide formed at the top or the bottom surface of the sheet-like substrate; and a control electrode set (4, 5, 54, 55, 56) for controlling light passing through the optical waveguide; wherein the control electrode set (4, 5, 54, 55, 56) comprises a first electrode set (4, 5) arranged on the sheet-like substrate and a second electrode set (54, 55, 56) arranged under the sheet-like substrate so that the sheet-like substrate is interposed between the first electrode set (4, 5) and the second electrode set (54, 55, 56); wherein the optical modulator device further comprises a supporting substrate arranged under the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) and bonded to the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) by means of an adhesive layer that is arranged between the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) and the supporting substrate; wherein the first electrode set (4, 5) comprises a set of coplanar electrodes (4, 5) which includes at least a signal electrode and a first ground electrode and the second electrode set (54, 55, 56) is arranged to include at least a second ground electrode so that an electric field is applied to the optical waveguide by means of the signal electrode and the first ground electrode of the first electrode set (4, 5) in cooperation with the second ground electrode (54, 55, 56) of the second electrode set (54, 55, 56); wherein a low refractive index layer is arranged on the lower surface of the sheet-like substrate between the sheet-like substrate and the second ground electrode (54, 55, 56) of the second electrode set (54, 55, 56) so as to reduce an electrode loss and to increase an optical confinement factor; wherein the low refractive index layer is an air or a resin layer, is wider than the signal electrode of the first electrode set (4, 5) and extends under the signal electrode along its entire length; and wherein the relation: d 2 < ε 2 / ε 1 × d 1 is satisfied, where ε 2 is the dielectric constant of the low refractive index layer, d 2 is the thickness of the low refractive index layer, ε 1 is the dielectric constant of the sheet-like substrate, and d 1 is the thickness of the sheet-like substrate. a sheet-like substrate which has a top surface and a bottom surface, exhibits an electro-optical effect and has a thickness of 10 μm or less; an optical waveguide formed at the top or the bottom surface of the sheet-like substrate; and a control electrode set (4, 5, 54, 55, 56) for controlling light passing through the optical waveguide; wherein the control electrode set (4, 5, 54, 55, 56) comprises a first electrode set (4, 5) arranged on the sheet-like substrate and a second electrode set (54, 55, 56) arranged under the sheet-like substrate so that the sheet-like substrate is interposed between the first electrode set (4, 5) and the second electrode set (54, 55, 56); wherein the optical modulator device further comprises a supporting substrate arranged under the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) and bonded to the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) by means of an adhesive layer that is arranged between the sheet-like substrate with the control electrode set (4, 5, 54, 55, 56) and the supporting substrate; wherein the first electrode set (4, 5) comprises a set of coplanar electrodes (4, 5) which includes at least a signal electrode and a first ground electrode and the second electrode set (54, 55, 56) is arranged to include at least a second ground electrode so that an electric field is applied to the optical waveguide by means of the signal electrode and the first ground electrode of the first electrode set (4, 5) in cooperation with the second ground electrode (54, 55, 56) of the second electrode set (54, 55, 56); wherein a low refractive index layer is arranged on the lower surface of the sheet-like substrate between the sheet-like substrate and the second ground electrode (54, 55, 56) of the second electrode set (54, 55, 56) so as to reduce an electrode loss and to increase an optical confinement factor; wherein the low refractive index layer is an air or a resin layer, is wider than the signal electrode of the first electrode set (4, 5) and extends under the signal electrode along its entire length; and wherein the relation: d 2 < ε 2 / ε 1 × d 1 is satisfied, where ε 2 is the dielectric constant of the low refractive index layer, d 2 is the thickness of the low refractive index layer, ε 1 is the dielectric constant of the sheet-like substrate, and d 1 is the thickness of the sheet-like substrate.