An electrode substrate includes an insulation substrate and a reference electrode on the insulation substrate. The reference electrode includes a silver layer, a first silver chloride layer, and a second silver chloride layer. The silver layer is on the insulation substrate. The first silver chloride layer is on the silver layer on the insulation substrate, and covers the silver layer. The second silver chloride layer is on the first silver chloride layer on the insulation substrate, and covers the first silver chloride layer. An area void fraction of the first silver chloride layer is larger than an area void fraction of the second silver chloride layer in any longitudinal section.