This invention prevents leakage of an internal liquid and breakage of a reference electrode resulting from volume expansion of the internal liquid due to the congelation of the internal liquid, and comprises a vessel that forms a sealed space, an internal liquid that is contained in the vessel, an internal electrode that is immersed in the internal liquid, an ion conduction part that is arranged on a wall of the vessel and that electrically connects the internal liquid and a measurement sample, and a volume expansion absorption mechanism 6 at least a part of which is immersed in the internal liquid and that absorbs volume expansion of the internal liquid due to congelation of the internal liquid.