We have developed a solid-type pH sensor for dairy wagyu-rumen measurements. The pH sensor is composed of a sensing electrode of indium tin oxide (ITO) film and a metal–oxide– semiconductor field-effect transistor (MOSFET). In the solid-state sensor structure, the ITO electrode is a separated gate of the FET and the ITO electrode can be patterned with a suitable capture structure for the wagyu-rumen environment. A sensor system with a compact size can be fabricated by eliminating the reference solution. The pH sensors have a sensitivity of 10–15 μA/pH under different sensing contact areas from 12 to 78 mm2. Long-term tests with interval experiments have been completed in 18 d, and the results demonstrate that the pH sensor has a stable performance for rumen pH measurements over a long time.