Thermosetting adhesive sheet and sub-gasket for fuel cell

A thermosetting adhesive sheet for sealing a 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 for a fuel cell, the thermosetting adhesive sheet comprising,

• an adhesive layer comprising a cured product of an adhesive composition containing
• a polyurethane resin having a reactive functional group, and
• a cross-linking agent, wherein the reactive functional group contains a carboxyl group and the cross-linking agent contains a multifunctional epoxy cross-linking agent,
• wherein the adhesive layer has:
• a gel fraction of 60 mass % or higher;
• a storage elastic modulus at 100° C. (G′100) of 5.0×104 Pa or higher and 1.0×108 Pa or lower;
• a decreasing rate of a storage elastic modulus at 120° C. (G′120) to the storage elastic modulus at 100° C. (G′100) of 0.5 or less, and the decreasing rate is defined as (G′100−G′120)/G′100;
• and
• a ratio (b/a) of molar amount (a) of the carboxyl group of the polyurethane resin to molar amount (b) of the epoxy group of the multifunctional epoxy cross-linking agent is 1.3 or higher and 10.0 or lower.