An electrode catalyst layer, capable of having high catalytic activity in a small thickness, for use in a polymer electrolyte fuel cell having an entangled (cobweb-like) structure. The electrode catalyst layer is produced through a process including a step of forming a thin film with a film-forming material containing a combination of platinum, oxygen, and nitrogen, a combination of platinum, oxygen, and boron, or a combination of platinum, oxygen, nitrogen, and boron, and a step of forming a catalyst material, which has the entangled structure and principally contains platinum as a main component by reducing the film-forming material.