We propose an electrical-optical converter using electric-field-coupled metamaterial antennas on an electro-optic (EO) modulator for wireless millimeter-wave/terra-hertz applications. By wireless signal irradiation to the proposed device, strong millimeter-wave/terra-hertz electric field can be induced on the electric-field-coupled metamaterial antennas. The induced electric field can be used for optical modulation through EO effects when a lightwave propagates into an optical waveguide located under regions of the induced millimeter-wave/terra-hertz electric field. Analysis in millimeter-wave and optical modulation of the proposed device are discussed in detail for operational frequency of 100 GHz. Device experiment in fabrication and preliminary measurement are also reported.