In a process of manufacturing a membrane electrode assembly, seal-material flow holes (62a, 62b) in the form of through-holes are formed, separately from manifold holes (16a-16f), in the membrane electrode assembly prior to injection molding. When the membrane electrode assembly is placed in a mold for injection molding, the seal-material flow hole (62a) is located in a cavity (44a). When a seal material is supplied from a supply port (42) formed at a location where the manifold hole (16a) is formed, the seal material that flows toward the upper die (40a) passes the seal-material flow hole (62a) in the cavity (44a), and then flows toward the lower die (40b), so as to reduce the unevenness between the amounts of supply of the seal material to the upper die (40a) and the lower die (40b).