1. Introduction In an international linear collider (ILC) plan, superconducting radio frequency (SRF) cavities are essential components for acceleration of the charged particles beam. The cavities are made of high purity niobium (Nb) and the inner surfaces of these cavities are required to be smooth enough because they play a crucial role in the performance of cavities. Typically, in order to obtain a good surface finish, electropolishing is used for the SRF cavities made of Nb. Typical electropolishing process uses the electrolyte composed of hydrofluoric acid (HF, about 55 mass%) and sulfuric acid (H2SO4, about 98 mass%) in a volumetric ratio of 1:91). However, there is a strong demand to replace the electrolyte with less hazardous one. In this study, we made a preliminary search for less hazardous electrolyte that could change the traditional HF-H2SO4 electropolishing system. 2. Experimental Following three types of electrolytes are tried for electropolishing of Nb: 3.7 M H2SO4, 10 M NaOH and 80 mass% CH3SO3H + 3 mass% NH4HF. A square of Nb (99% +, 20×20×0.5 mm3, Nilaco) is fixed on a Teflon holder (with exposed area of 1.13 cm2) and used as a working electrode. Before electropolishing, Nb samples is polished up to #6/0 SiC paper. A plate of aluminum is used as a counter electrode except in a NaOH solution where a nickel plate is used as a counter electrode. Electropolishing is carried out under constant voltage (5 – 10 V) for 30 min at temperatures of 30 - 80ºC. In each solution, I–V character and mass change are evaluated. The Nb surface after the electropolishing is characterized by a surface roughness meter and FE-SEM. 3. Experimental results and discussion In a 3.7 M H2SO4 solution at 30ºC, a blue oxide film was formed on Nb surface after applying 5 and 10 V. The results indicated that no polishing of Nb would proceed in this solution in a direct current mode. In a 10 M NaOH with constant voltages of 5 and 10 V, a gray-white oxide film was formed on Nb surface but no polishing proceeded either below 50ºC. At 80 ºC in a 10 M NaOH, a remarkable mass loss was observed at 10 V but the resultant surface was not very uniform. In a 80 mass% CH3SO3H + 3 mass% NH4HF at 30ºC, a gray-white oxide film formed on Nb surface at a constant voltage of 1 V. As the voltage is increased to 5 V, polishing proceeded most efficiently with mass loss of 4.3 mg cm-2 in 30 min. Further increase in voltage resulted in reduction in mass loss. The roughness of Nb surface was apparently improved in this solution. References 1) P. V. Tyagi, M. Nishiwaki, T. Noguchi, M. Sawabe, T. Saeki, H. Hayano, S. Kato, Applied Surface Sci., 285P, 778 (2013). 2) Japan patent JP2008-95192A Acknowledgment The authors are grateful to Tohoku Business Federation for their financial support.