A reformer includes a reforming chamber having a raw fuel passage through which a raw fuel flows, the reforming chamber being filled with or carrying a reforming catalyst, a supply chamber disposed upstream of the reforming chamber, for uniformly supplying the raw fuel to the raw fuel passage, and a discharge chamber disposed downstream of the reforming chamber, for uniformly discharging the raw fuel from the raw fuel passage. The raw fuel passage has first and second reversers for reversing the direction in which the raw fuel flows. The raw fuel passage has a cross-sectional area which is smaller in a downstream portion thereof than in an upstream portion thereof.