NOVELTY - A device (39) restrains a first member on one end of an assembly including unit cell stack, first and second end plates, and at least one auxiliary plate (33, 34), and a second member on other end. Screw (35) is fitted in auxiliary plate such that the screw end contacts the first end plate. Compressive device compresses the unit cell stack when the screw is fitted in auxiliary plate threaded hole.    USE - For portable power sources, electric vehicle power sources and domestic co-generation systems.    ADVANTAGE - The fuel cell has a small sized, simply constructed unit cell stack fastening mechanism. Creep deformation is reduced due to long-term pressure application.    DETAILED DESCRIPTION - The polymer electrolyte fuel cell comprises:    a fuel cell stack, each cell comprising a polymer electrolyte membrane, anode and cathode arranged across each membrane, an anode side conductive separator plate having a gas fuel supply path to the anode, and a cathode side conductive separator plate having oxidant gas supply path to the cathode;    first end plate located on one end of the unit cell stack;    second end plate located on the other end of the unit cell stack;    an auxiliary plate located at least outside the first end plate;    at least one restraining device having band shape to restrain a first member located on one end of assembly including unit cell stack, first and second end plates and auxiliary plate and a second member on the other end to restrict separation of the first and second members;    a screw fitted in a threaded hole formed in the auxiliary plate such that the end of the screw contacts the first end plate; and    compressive device generating a repulsive force to compress the unit cell stack when the screw is fitted into the threaded hole of the auxiliary plate.    The auxiliary plate is made of metal having elasticity and also functions as the compressive device.    INDEPENDENT CLAIMS are given for polymer electrolyte fuel cells.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the polymer electrolyte fuel cell.    Unit cell stack (30)    First end plate (31)    Second end plate (32)    Auxiliary plate (33, 34)    Screw (35)    Restraining device (39)