To improve the speed matching of light waves and microwave signals without increasing the thickness of electrodes by covering signal electrodes holding the gap of an air layer with GND electrodes and decreasing the dielectric constant.

Optical waveguides 20 (21, 22) are formed by thermal diffusion of Ti on an LiNbO3 substrate 10 having an electro-optic effect. Buffer layers 31, 32 are formed of SiO2, etc., on the LiNbO3 substrate 10 and further, progressive wave electrodes (coplanar wave guides) composed of the signal electrode 40 and the GND electrode 50 are formed as the progressive wave electrodes on the buffer layers 31, 32. A shielding conductor 70 is fixed atop the progressive wave electrodes. The GND electrode 50 is formed higher than the signal electrode 40. The gap of the air layer is formed between the signal electrode 40 and the shielding conductor 70. This air layer is set at a suitable thickness and the shielding conductor 70 is brought near to the electrode 40, by which the number of the electric lines of force of the peripheral space is increased and the dielectric constant is effectively lowered.