NOVELTY - The resin composition is three-dimensionally crosslinked by addition polymerizing a sealing composition which comprises (A) an addition polymerizable oligomer in which the main chain of the molecule has a linear polyisobutylene structure or perfluoropolyether structure and which has an alkenyl group at least at each end, (B) a hardener containing at least 2 hydrogen atoms per molecule each bonded to a silicon atom, and (C) a hydrosilylation catalyst.    USE - Used in sealing fuel cells for motor vehicles.    ADVANTAGE - Each cell can be reduced in thickness and size to enable the fuel cell assembly to have a high output. The fuel cell is made airtight.    DETAILED DESCRIPTION - Sealing separators of each cell of a fuel cell uses a pair of electrodes and solid electrolyte ion exchange resin obtained by three-dimensionally crosslinking a low gas-permeable, reactive liquid resin composition. The resin composition is three-dimensionally crosslinked by addition polymerizing a sealing composition which comprises (A) an addition polymerizable oligomer in which the main chain of the molecule has a linear polyisobutylene structure or perfluoropolyether structure and which has an alkenyl group at least at each end, (B) a hardener containing at least 2 hydrogen atoms per molecule each bonded to a silicon atom, and (C) a hydrosilylation catalyst. The solid electrolyte ion exchange resin is contacted with the separators or electrodes.    An INDEPENDENT CLAIM is also included for a sealing agent using the sealing method in which when the main chain of the oligomer is a polyisobutylene structure, the main chain of the hardener is a polyisoprene structure, and when the main chain of the oligomer is a perfluoropolyether structure, the main chain of the hardener is a perfluoropolyether structure.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section of each cell of a fuel cell.    Cell (20)    Ion Exchange Resin (21)    Electrodes (22, 23)    Seal Separators (24, 25)    Resin Composition (26)