In an electrode unit, a working electrode and a counter electrode are disposed in a cylindrical section having a cylindrical shape, which is a sensor main body, a distal end face of the working electrode is covered with a diaphragm that allows liquid to be measure to permeate, and an internal liquid chamber in which internal liquid of an electrolyte is stored and a standard solution chamber in which a standard solution for calibration is stored are partitioned by the diaphragm. The standard solution chamber is surrounded by an inner wall of the cylindrical section and a cap. When the electrode unit is mounted on a medical solution tank, zero adjustment is automatically performed using a standard solution inside the electrode unit. Thereafter, when the cap is opened, the standard solution is discharged and measurement in an actual use can be performed.
