[en] Marui Galvanizing Co., Ltd. started development of internal surface electropolishing technology for niobium cavities, making use of years of experience in electropolishing construction. Focusing on vertical electrolytic polishing (VEP) method, which is superior in cost and suitable for mass production, we have been developing technology and equipment jointly with KEK. We succeeded in greatly improving the uniformity of the removal thickness, which was a major problem of VEP, by developing our unique Ninja cathode and optimizing the VEP parameters. And, in the VEP of the niobium single cell cavity, an accelerating gradient of 32 MV/m (Q₀=8.0E9), comparable to that of the horizontal electrolytic polishing (HEP) was achieved. Also, in the niobium 9 cell cavity, in addition to the improvement of the Ninja cathode, the flow of the bubbles in the VEP is controlled to improve the distribution of removal thickness and the polishing state, and the accelerating gradient of 28 MV/m (Q₀ = 6.7E9) similar to HEP is achieved. In this presentation, we will report on the results of these VEP technology developments and future efforts. (author)