A liquid-contact portion exposed into a measurement tube is provided, and a main body portion formed by covering a base material with a conductor is provided. A terminal portion electrically connected to the conductor is provided. The main body portion includes a small diameter portion having a cylindrical shape and being inserted at one end thereof into an electrode insertion hole of the measurement tube, and the one end corresponds to the liquid-contact portion; and a large diameter portion having a disc shape extending outward from the other end of the small diameter portion in a radial direction. The large diameter portion includes an annular groove opened toward inside a measurement, tube, and cutaway's extending outward from the annular groove in the radial direction and opening to an outer peripheral surface of the large diameter portion.