A novel pH microsensor with a built-in reference electrode is proposed, and its feasibility is demonstrated to pursue a pH microsensor fully integrated with a miniaturized reference electrode. An iridium-oxide electrode and a pH-insensitive field-effect transistor are used as a pH sensor and a reference electrode, respectively. An Iiridium-oxide layer and a pH-insensitive membrane are formed on a single piece of indium-tin-oxide-coated glass by using the simple fabrication processes. The sensitivity of the proposed device is 62.5 mV/pH and matches well with the estimated value within a 0.8 mV/pH deviation. with the proposed system, the strict electrical requirements of a reference field-effect transistor in conventional differential measurement system can be circumvented.