In this paper we discuss the influence of Na₂SO₄ EO water (electrolyzed oxidizing water) on the surface of oxygen-free copper. First, we threw light on the etching efficiency of Na₂SO₄ EO water on the surface of oxygen-free copper by comparing it with the etching efficiency of soaking in NaCl EO water or chemicals. The comparative study showed that as for the etching efficiency on oxygen-free copper, Na₂SO₄ EO water was superior to chemicals, while inferior to NaCl EO water. Next, we threw light on the influence of Na₂SO₄ EO water on the surface shape of oxygen-free copper by the SEM observations and AFM measurement. As the result, the effect of Na₂SO₄ EO water on oxygen-free copper, which made its surface coarse, turned out weaker than that of NaCl EO water. Lastly, by analyzing the surface, we made it clear that Na₂SO₄ EO water did not form foreign substances such as copper chloride which was formed when using NaCl EO water. Unlike NaCl EO water, Na₂SO₄ EO water can be utilized widely for surface washing or surface treatment of precision machine.