A pH sensor includes a glass electrode unit to respond to pH; a comparative electrode unit to be measured for its potential relative to the glass electrode unit; a main pipe to convey test liquid; and a fluid inlet connected to the glass electrode unit and the main pipe. The glass electrode unit includes: a first storage unit filled with electrolyte solution; a first electrode to electrically connect the inside and outside of the first storage unit; a glass film disposed between the first storage unit and the main pipe; and a first pressure transmission unit connected to the fluid inlet. The first pressure transmission unit regulates the internal pressure of the first storage unit to control the pressure difference between the first storage unit and the main pipe separated by the glass film. The comparative electrode unit includes: a second storage unit filled with buffer solution; a second electrode to electrically connect the inside and outside of the second storage unit; a liquid junction unit disposed between the second storage unit and the main pipe; and a second pressure transmission unit connected to the fluid inlet. The second pressure transmission unit regulates the internal pressure of the second storage unit to control the pressure difference between the second storage unit and the main pipe separated by the liquid junction unit.