This invention provides a fuel cell electrode catalyst in which at least one transition metal element and at least one chalcogen element are supported on a conductive support, wherein the fuel cell electrode catalyst comprises a core portion comprising a transition metal crystal and a shell portion comprising surface atoms of the transition metal crystal particle and chalcogen elements coordinating to the surface atoms, and the outer circumference of the core portion is being partially covered with the shell portion. The fuel cell electrode catalyst has a high level of oxygen reduction performance, high activity as a fuel cell catalyst and comprises a transition metal element and a chalcogen element.