A novel electro-optic (EO) microwave-lightwave converter using a patch antenna embedded with a narrow gap is proposed and experimentally demonstrated. By embedding a narrow gap at the center of the patch antenna and an optical waveguide located under the gap, a wireless microwave signal can be received and converted to an optical signal directly through EO modulation of a lightwave propagating in the optical waveguide. In the experiment, optical sidebands were observed clearly with the irradiation of an 18GHz wireless signal to the fabricated prototype device. This device operates with no external power supply and no additional modulation electrode. Therefore, it can be operated with low microwave distortion.