For ensuring the long-term performance of an Ag/AgCl reference electrode, a calibration technique was developed by using the linear relationship between the electrical conductivity and the KCl concentration of an internal electrolyte. It was observed that the electrical conductivities increased with an increase of the KCl concentration of the internal electrolyte, and the potentials of the Ag/AgCl electrode shifted anodically with a decrease of the KCl concentration. For simulating a long-term exposure of the electrode, we designed a reference electrode with a tiny perforation to accelerate the diffusion of the internal electrolyte. During the acceleration tests, the potentials of the Ag/AgCl electrode were shifted by almost the same amount as the potentials calculated from the change of the electrical conductivity of the internal electrolyte. Consequently, this technique was confirmed to be applicable for the calibration of an Ag/AgCl electrode, especially when exposed to non-electrolytic aqueous solutions.