There is provided a fuel cell unit including at least: a membrane electrode assembly including an electrolyte membrane and two catalyst layers sandwiching the electrolyte membrane therebetween; two gas diffusion layers sandwiching the membrane electrode assembly therebetween; an oxygen supplying layer brought into contact with one gas diffusion layer of the two gas diffusion layers; two collectors; and a seal portion, in which: the fuel cell unit has side surfaces of which a side surface parallel to a proton conductive direction of the electrolyte membrane has an opening portion provided in a part of the side surface; and a part of the one gas diffusion layer brought into contact with the oxygen supplying layer constitutes a part of an outer surface of the fuel cell unit. The fuel cell unit can enhance drainage efficiency and achieve effective supply of an oxidizer.