A desulfurizer includes a filled chamber having a raw fuel passage through which a raw fuel flows, the filled chamber being filled with a desulfurizing agent, a supply chamber disposed upstream of the filled chamber, for uniformly supplying the raw fuel to the raw fuel passage, and a discharge chamber disposed downstream of the filled 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.