THERMOSETTING ADHESIVE SHEET AND SUB-GASKET FOR FUEL CELL

A thermosetting adhesive sheet for sealing the periphery of a membrane electrode assembly, which is composed of a solid polymer electrolyte membrane and electrodes placed on both sides of the solid polymer electrolyte membrane contains an adhesive layer . The adhesive layer is formed of a cured product of an adhesive composition containing a polyurethane resin having a reactive functional group and a cross-linking agent, where the adhesive layer has a gel fraction of 60 mass % or higher, a storage elastic modulus at 100° C. (G′) of 5.0×10Pa or higher and 1.0×10Pa or lower, and a decreasing rate of a storage elastic modulus at 120° C. (G′) to the storage elastic modulus at 100° C. (G′) of 0.5 or less.