A perovskite-type oxide-based electrode showed good properties of amperometric sensing to hydrogen-phosphate ion. The anodic current of carbon-based electrodes loaded with La-Co-based perovskite-type oxide, the sensing signal, increased with increasing concentration of hydrogen-phosphate ion at +0.85 to +1.0 V vs, a saturated calomel electrode. The carbon electrode loaded with La0.9Ce0.1CoO3 showed remarkable selectivity to HPO42- among the examined anions of F-, Cl-. Br-, SCN-, NO3-, SO42-, CO32-, and ClO4-, although it received serious interference from I-. The use of a thin film electrode of perovskite-type oxide drastically improved amperometric sensing to hydrogen-phosphate ion. The LaCoO3 thin film sensor device responded to HPO42- at concentrations between 1.0 x 10(-6) and 1.0 x 10(-1) M, and the 90% response lime was as short as ca. 5 min or ca. 10 s at the low or high concentration range of hydrogen-phosphate ion, respectively. The LaCoO3 thin film sensor device showed the highest selectivity to HPO42- among the examined anions.