Nii, K.N. Carbonnier, P. Chouhan, V. Gasser, Y. Hayano, H. Ida, Y.I. Kato, S. Maurice, L. Monjushiro, H. Proslier, Th. Saeki, T. Sawabe, M. Servouin, C. Yamaguchi, T.Y. Éozénou, F. JACoW Geneva, Switzerland 978-3-95450-191-5 10.18429/JACoW-SRF2017-TUPB093 English 612-615 TUPB093 ion cavity cathode experiment accelerating-gradient Contribution to a conference proceedings 2018 2018-01 https://doi.org/10.18429/JACoW-SRF2017-TUPB093 http://jacow.org/srf2017/papers/tupb093.pdf Marui Galvanizing Co. Ltd. has been improving Vertical Electro-Polishing (VEP) technology for Nb superconducting RF cavity in collaboration with KEK. In this collaboration, we developed a unique cathode namely Ninja cathode for VEP treatment of Nb cavities. We have already reported that longitudinal symmetry in niobium removal and surface state of a single cell cavity were improved after VEP using the Ninja cathode. In this article, we report a result of accelerating gradient evaluation for 1.3 GHz single cell RF cavity after VEP with Ninja cathode in collaboration with KEK and CEA Saclay.