Provided is a sealant for use in contact with a polyelectrolyte membrane; at least the sealant surface contacting the polyelectrolyte membrane is constituted of a resin composition containing a crystalline polyester resin as a main component; the resin composition has a storage elastic modulus at 40 DEG C of 2.5 MPa or more and 5 MPa or less; the resin composition further contains an epoxy resin and an isocyanate-based crosslinking agent; and the crystalline polyester resin is crosslinked with the isocyanate-based crosslinking agent.