NOVELTY - The assembly (12) has a solid polymer electrolyte membrane (24) sandwiched between an anode (26) and a cathode (28). The anode and cathode contain electrode catalyst layers (30, 38), gas diffusion layers (32, 40) and porous layers (34, 42) interposed between the catalyst and diffusion layers. A first piled body (35) containing one of the diffusion layers and porous layers defines percolation pressure higher than a second piled body (43) containing other diffusion layer and porous layer, where the first and second piled bodies define percolation pressure of 25 to 120 kPa and 5 to 25 kPa.    USE - Membrane electrode assembly for use in a solid polymer fuel cell for a vehicle.    ADVANTAGE - The assembly increases cell voltage to 0.70 V or more when the first piled body defines percolation pressure of 120 kPa or less and the second piled body defines percolation pressure of 5 to 25 kPa so as to achieve excellent power generation property of the fuel cell such that power generation reactions can be efficiently carried out, while preventing flooding even under higher gas pressure when the piled bodies define percolation pressures within the above ranges, so that thickness dispersion of the electrolyte membrane can be reduced, thus effectively preventing physical deformation of the electrolyte membrane and readily transferring components between the anode and the cathode, and hence efficiently carrying out electrode reactions. The assembly prevents deterioration of the electrolyte membrane and improves gas barrier property against reactant gases and durability of the assembly. The assembly prevents generated water from being excessively transferred from the electrode catalyst layer to the gas diffusion layer and from excessively remaining around the electrode catalyst layer and the gas diffusion layer, so that water retention and water discharge properties can be well balanced between the anode and the cathode.    DESCRIPTION OF DRAWING(S) - The drawing shows a longitudinal sectional schematic view of a principal part of a fuel cell containing a membrane electrode assembly.    Membrane electrode assembly (12)    Solid polymer electrolyte membrane (24)    Anode (26)    Cathode (28)    Electrode catalyst layers (30, 38)    Gas diffusion layers (32, 40)    Porous layers (34, 42)    Piled bodies (35, 43)