An optical modulator device for controlling light which passes through an optical waveguide comprising: a sheet-like substrate which has an electro-optical effect and has a thickness of 10 μm or less; the optical waveguide formed on the sheet-like substrate; and a control electrode set (4, 5, 54, 55, 56) for controlling light passing through the optical waveguide; wherein the optical device comprises a modulation region in which the control electrode set (4, 5, 54, 55, 56) comprises a first electrode set (4, 5) and a second electrode set (54, 55, 56) which are arranged so that the sheet-like substrate is interposed between the first electrode set (4, 5) and the second electrode set (54, 55, 56), and in which the first electrode set (4, 5) includes coplanar electrodes (4,5) which are arranged on the sheet-like substrate and comprise at least a signal electrode and a first ground electrode and the second electrode set under the sheet-like substrate is configured to include at least a second ground electrode so as to apply an electric field to the optical waveguide in cooperation with the signal electrode of the first electrode set (4, 5); characterized in that the sheet-like substrate is arranged on an intervening layer which has a low dielectric constant and is arranged on a supporting substrate; the optical modulator device further comprises a signal input/output region in which a microwave modulating signal may be input to the optical modulator or be output from the optical modulator, wherein, either, the second electrode set (54, 55, 56) is not present in at least a region below the signal electrode of the first electrode set (4, 5); or, the second electrode set is disposed between the intervening layer having the low dielectric constant and the supporting substrate. a sheet-like substrate which has an electro-optical effect and has a thickness of 10 μm or less; the optical waveguide formed on the sheet-like substrate; and a control electrode set (4, 5, 54, 55, 56) for controlling light passing through the optical waveguide; wherein the optical device comprises a modulation region in which the control electrode set (4, 5, 54, 55, 56) comprises a first electrode set (4, 5) and a second electrode set (54, 55, 56) which are arranged so that the sheet-like substrate is interposed between the first electrode set (4, 5) and the second electrode set (54, 55, 56), and in which the first electrode set (4, 5) includes coplanar electrodes (4,5) which are arranged on the sheet-like substrate and comprise at least a signal electrode and a first ground electrode and the second electrode set under the sheet-like substrate is configured to include at least a second ground electrode so as to apply an electric field to the optical waveguide in cooperation with the signal electrode of the first electrode set (4, 5); characterized in that the sheet-like substrate is arranged on an intervening layer which has a low dielectric constant and is arranged on a supporting substrate; the optical modulator device further comprises a signal input/output region in which a microwave modulating signal may be input to the optical modulator or be output from the optical modulator, wherein, either, the second electrode set (54, 55, 56) is not present in at least a region below the signal electrode of the first electrode set (4, 5); or, the second electrode set is disposed between the intervening layer having the low dielectric constant and the supporting substrate.