Magnesium (Mg) is widely used as the automobile parts and biomedical materials because it is lightweight and high strength material. However, it is well known that Mg has large corrosion rate, which limits the application of Mg. The hydrogen evolution rate on Mg increases with noble potential from corrosion potential. This phenomenon is called the Negative Difference Effect (NDE).1-8) The effect of pH on the Mg dissolution and hydrogen evolution during the anodic dissolution is one of the key issue for understanding of NDE phenomenon.4) In the present study, we developed the pH sensing electrochemical cell to investigate the pH changes near the Mg electrode due to the anodic dissolution. Schematic of the pH sensing electrochemical cell developed in the present study is shown in Fig. 1. The electrochemical cell was constructed using transparent acrylic plates. The working electrode and the counter electrode were placed in compartments separated by a glass filter to avoid the pH changes due to the gas generation on each electrode. Tungsten/tungsten oxide electrode was set closed to the working electrode. Tungsten/tungsten oxide electrode was prepared by the anodic oxidation. The diameter of tungsten electrode was 3 mm. The anodic oxidation of tungsten electrode was performed at 0.52 V vs. SSE in 1 mol dm-3 phosphate buffer solution (pH=1) for 1 min. This electrode was immersed in the buffer solution adjusted to arbitrary pH to evaluate the performance as the pH sensor. The distance between the tungsten/tungsten oxide electrode and the working electrode was 1 mm. The anodic polarization curve measurement of Mg was conducted by three electrode system. The working electrode was the Mg electrode and the counter electrode was Pt wire. The saturated potassium chloride/silver/silver chloride electrode (SSE) was used as the reference electrode. The 1 mol dm-3 sodium sulfate solution was used as the electrolytic solution. The pH was monitored using the tungsten/tungsten oxide electrode during the anodic polarization curve measurement of Mg electrode. The relation between pH and anodic current of Mg was discussed in the present study. Reference [1] G. Song, A. Atrens, D. Stjohn, J. Nairn, and Y. Li, Corros. Sci., 39, 855 (1997). [2] R.L. Petty, A.W. Davidson, J. Am. Chem. Soc., 76, 363-366 (1954). [3] G. Song, A. Atrens, D. Stjohn, X. Wu, and J. Nairn, Corros. Sci., 39, 1981 (1997). [4] L. Rossrucker, A. Samaniego, J.-P. Grote, A. M. Mingers, C. A. Laska, N. Birbilis, G. S. Frankel, K. J. J. Mayrhofer, J. Electrochem. Soc., 162, C333-C339 (2015). [5] G. S. Frankel, A. Samaniego, and N. Birbilis, Corros. Sci., 70, 104 (2013). [6] G. Williams, N. Birblis, and H. N. McMurray, Electrochem. Commum., 36, 1 (2013). [7] G. L. Song and A. Atrens, Ave. Eng. Mater., 1, 11 (1999). [8] G. L. Song and A. Atrens, Ave. Eng. Mater., 5, 837 (2003). Figure 1