Horizontal slot waveguides enable light to be strongly confined in thin regions. The strong confinement of light in the slot region offers the advantages of enhancing the interaction of light with matter and providing highly sensitive sensing devices. We theoretically investigated fundamental characteristics of horizontal slot waveguides using Nb₂O₅. The coupling coefficient between SiO₂ slot and air slot waveguides was calculated. Characteristics of bending loss in slot waveguide were also analyzed. The etching conditions in reactive ion etching needed to obtain a sidewall with high verticality were studied. We propose a process for fabricating horizontal slot waveguides using Nb₂O₅ thin film deposition and selective etching of SiO₂. Horizontal slot waveguides were fabricated that had an SiO₂ slot of less than 30 nm SiO₂. The propagated light passing through the slot waveguides was also obtained.