To utilize diamond color centers as high-fidelity functional quantum nodes, an optical interface that can efficiently interact with photons propagated through optical fibers is required. We propose an optical interface that combines an ultra-high-quality factor (Q>10^8) silica micro-toroid cavity with a nanocrystal embedded with a color center. Through simulations, we have confirmed the achievement of strong coupling between a nitrogen-vacancy center and the micro-toroid cavity within the proposed interface. To implement this interface, we have developed a fabrication process that enables the transplantation of a single diamond nanocrystal into the silica micro-toroid cavity. Furthermore, we have experimentally verified the fluorescence of diamond color centers within the silica. If the proposed interface is implemented, it is expected that optical signals can be obtained through optical fibers without losses. This opens up the possibility of utilizing diamond color centers for high-fidelity quantum network nodes and highly sensitive quantum sensors.