In this study, we propose a novel silicon-core optical fiber with a bowtie-shaped slot core structure for ultrasmall light-spot transmission. Our simulations show that this dielectric structure creates a nanosized optical spot with the high intensity and transmits it with the low loss. As an example, we obtained an optical spot with the full width at half maximum of 5 nm × 5 nm and a peak power density 167 times higher than that of the surrounding areas. The optical loss because of the scattering in the waveguides and the material absorption was estimated to be 0.58 dB/cm, which is a thousand times less than the optical losses in typical plasmonic waveguides. We believe our proposed structure will contribute to research studies in the field of near-field sensing systems. It also has application potential in nanolithography with high-power lasers.