The invention relates to a tin oxide, an electrode catalyst for fuel cells, a membrance electrode assembly, and a solid polymer fuel cell. The tin oxide includes at least one element A selected from the group consisting of tantalum, tungsten, niobium, and bismuth, and antimony. Suitably, the at least one element A selected from the group consisting of tantalum, tungsten, niobium, and bismuth, andantimony are dissolved in solid solution in the tin oxide. It is also suitable for the value of [mol number of the element A/mol number of antimony] which is the ratio of the mol number of the elementA to the mol number of antimony to be 0.1 to 10.