A polysulfone is provided with a nitrogen-containing functional group having an affinity to an acid, an electrolyte membrane using the polysulfone, and a fuel cell including the electrolyte membrane. In particular, the polysulfone includes a nitrogen-containing functional group that has an affinity to an acid, such as a phosphoric acid, thereby having an excellent acid retaining ability. In an electrolyte membrane including the polysulfone and an acid, the amount of the retained acid can be controlled. Therefore, the electrolyte membrane has a high ionic conductivity and a high mechanical strength. A polysulfone blend of polysulfone and a thermoplastic resin prevents the dissolution of polysulfone by phosphoric acid, so that an electrolyte membrane using the polysulfone blend has an improved durability. A cross-linked reaction product of polysulfone, a cross-linking agent and a polymerization product of polysulfone, a thermoplastic resin, and a cross-linking agent strongly resist a phosphoric acid. Thus, a fuel cell using these polymerization products has a long lifetime and can operate at a high temperature and at a high efficiency.