To improve wireless communications systems, it is desirable to seamlessly combine fiber-based optical and terahertz wireless communications systems. In this paper, we demonstrate a waveguide optically driven terahertz wave modulator using a ring-shaped microstripline with a GaInAs photoconductive mesa structure. In this device, terahertz wave modulation is performed by controlling the photo-generated carriers in the GaInAs mesa. As a result, a maximum extinction ratio of 16.8 dB in the terahertz band was obtained with a light irradiation of 15 dBm (32 mW), and the 3 dB modulation bandwidth was approximately 170 MHz.