To clarify the reaction characteristics of electropolishing of SUS304 in phosphoric acid electrolyte, the polishing amount was determined by changing the voltage. Effects of varying the water content in the electrolyte were also clarified. Furthermore, we measured the oxygen gas on the anode side and hydrogen gas on the cathode side generated by the water splitting reaction that occurred in parallel with the electropolishing reaction. Using Ion Chromatography with Mass Spectrometry, we then analyzed the electrolyte chemical compositions before and after electropolishing. Results of these investigations demonstrate that the electropolishing amount increased as the voltage increased, but the electropolishing amount at each voltage decreased considerably as the water content decreased from 15% to 9.8% and 6.4%. Results show that, during electropolishing, the dimer and tetramer formation reaction of phosphoric acid proceeded in the electrolyte. This tendency is more pronounced when the water content is low. On the other hand, the amount of gas generated on the anode and cathode sides during electropolishing did not change with the change in water content. These phenomena can be explained by the water supply chain as follows. During electropolishing, the water splitting reaction proceeds at a steady rate and localized water shortage occurs on the electrode surface. To compensate for this water shortage, the lower the water content in the electrolyte, the more the phosphoric acid condensation reaction, a water-generating reaction, proceeds in preference to the phosphoric acid anionization reaction. As a result, the elution reaction of metals, the counter-reaction of the phosphoric acid anionization reaction, is suppressed in electrolytes with low water content.