Abstract: [Problems] To provide a method for desulfurization with a desulfurizing agent, which can efficiently desulfurize, for example, a hydrocarbon oil as a raw material and fuel for generating hydrogen for use in fuel cells, particularly kerosenes and light oils, without the need to use reduction treatment and hydrogen and at a temperature from room temperature to about 100° C., and a fuel cell system using the desulfurization method. There are also provided a solid acid useful as a constituent of the desulfurizing agent for use in the desulfurization method, and a process for producing the solid acid. [Means for Solving Problems] A solid acid comprising 20 to 99.9% by mass of aluminum oxide and 0.10 to 3.00% by mass of sulfur and having a specific surface area of not less than 150 m2/g and a pore volume of not less than 0.35 ml/g, and an I1540/I1450 ratio of 0.