A fuel cell (A1) includes a cell stack (B) and a casing (210) for housing the cell stack (B), and is supplied with two reactant gases flowing separately from each other. The cell stack (B) includes a plurality of solid electrolyte fuel cell units (200) stacked on one another with inter-unit spaces provided therebetween. One of the reactant gases is supplied to the inter-unit spaces and used for power generation. The casing (210) includes a peripheral wall (222) surrounding the cell stack (B). The peripheral wall (222) is provided with at least one gas inlet opening (223) for introducing the one of the reactant gases into the inter-unit spaces and at least one gas outlet opening (224) for discharging the introduced reactant gas, wherein total opening width dimension of the gas inlet opening (223) is greater than total opening width dimension of the gas outlet opening (224).