A small-sized fuel cell without performance deterioration is provided by a fuel cell unit including: a membrane electrode assembly which has an anode electrode and a cathode electrode provided on both sides of an electrolyte membrane; a diffusion layer for supplying the cathode electrode with air, disposed at the cathode electrode side of the membrane electrode assembly; and an air introducing layer for supplying the diffusion layer with the air, wherein when a total effective area of the electrolyte membrane at the cathode electrode side is A cm2, an average current density in an operation state of the fuel cell is I A/cm2, and a total area of the air introducing inlet is S cm2, a relation of A×I×0.5