We have proposed and developed a new wireless-optical signal converter utilizing stacked-patch antennas with a narrow gap and electro-optic (EO) crystal films for millimeter-wave radio-over-fiber systems. This converting device utilizes a structure of vertically stacked-patch antennas like the Yagi–Uda antenna, with a hybrid substrate composed of EO crystal films and low-dielectric-constant material plates. The electric field for wireless-optical signal conversion and the size of the patch antenna are increased by introducing the antenna and substrate stacked structure. Therefore, the conversion efficiency from wireless to optical signals can be improved. In the analysis by use of HFSS, the operational frequency was shifted to the higher range by 7Â GHz, and the optical modulation index was improved by 3.2Â dB with the double-stacked structure. In the experiments, the operational frequency shift by 8Â GHz and the modulation index improvement by 3.0Â dB were verified successfully, and the measured frequency response and directivity were in good agreement with the designed ones. The conversion of wireless 60Â GHz band signals with QPSK modulation and the transfer of the converted signal over 1Â km long fiber were also demonstrated.