To provide an optical modulator for modulating light by microwaves in an electromagnetic wave region including in principle a THz region by utilizing interaction of the electron beam radiated from an electron radiation section and traveling in a vacuum, and the light oozed into the vacuum from a dielectric optical waveguide of a three-layered structure.

The dielectric waveguide 12 formed in the travel direction of an electron beam radiated from an electron radiation section 11 includes three or more dielectric layers including a first layer 12a, a second layer 12b. and a third layer 12c laminated successively from an electron beam traveling path side and a metal layer 12d. A part of the light input from a light input end 12e of the first layer 12a, a part of the microwave input from a microwave input end 12f of the third layer 12c, and the electron beam intersect with each other, so that an electric field component of the light and the microwave is generated. The thicknesses of the first , second, and third layers are set so that the traveling speed of the electron beam becomes equal to the traveling speed of light and the microwaves in the electron beam traveling direction.