In an electrode unit 80, a working electrode 88 and a counter electrode 90 are disposed in a cylindrical section 83 having a cylindrical shape, which is a sensor main body, a distal end face of the working electrode 88 is covered with a diaphragm 91 that allows liquid to be measure to permeate, and an internal liquid chamber 92 in which internal liquid of an electrolyte is stored and a standard solution chamber 93 in which a standard solution Lc for calibration is stored are partitioned by the diaphragm 91. The standard solution chamber 93 is surrounded by an inner wall of the cylindrical section 83 and a cap 84. When the electrode unit 80 is mounted on a medical solution tank 70, zero adjustment is automatically performed using a standard solution inside the electrode unit 80. Thereafter, when the cap 84 is opened, the standard solution is discharged and measurement in an actual use can be performed.
