[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 0.12 or less, wherein I1450 represents a peak height of a peak (1450±5 cm−1) attributable to a Lewis acid site as measured by
pyridine adsorption
Fourier transform infrared spectroscopy, and I1540 represents a peak height of a peak (1540±5 cm−1) attributable to a Broensted acid site, and a process for producing the
solid acid, a method for desulfurizing a
hydrocarbon oil using the
solid acid as a desulfurizing agent, and a fuel
cell system using the desulfurization method.