Drift has been a problem for rumen pH sensors, but it is not known whether the reference electrode or the indicating electrode of the pH sensor is the main cause of the drift. In this study, the drift of each electrode was evaluated by placing pH sensors in a cow’s rumen and temporarily removing them at intervals over a four-month period. Our experiment revealed that the drift of the indicating electrode caused by changes in sensitivity is the critical drift issue in realizing a stable rumen pH sensor, although the drift is caused by both the reference electrode and the indicating electrode. The sensitivity of the indicating electrode was around 57.0 mV/pH unit at the start of the experiment but dropped to 37.1 and 49.0 mV/pH unit for Cow1 and Cow2, respectively, after placing the pH sensors in the rumen for four months. By assuming a change in the rumen’s pH of around 2 pH units, the maximum drift caused by the indicating electrode appears to be 0.68 pH unit. From the observation of the indicating electrode, we found that the drift was caused by impurities on the glass surfaces, and the pH sensitivity was recovered by removing these impurities. Because the glass electrode cannot be cleaned when in the cow’s rumen, the development of a glass electrode with resistance to impurities is required for rumen pH sensors.