TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: Each porous supporting carbon body comprises carbon cloth. The first and second porous carbon layers comprise first carbon cloth and second carbon cloth made of first and second threads, respectively. The first carbon cloth has finer mesh than that of the second carbon cloth. The first porous carbon layer comprises carbon unwoven fabric and the second porous carbon layer comprises carbon cloth. The first and second porous carbon layers may comprise first and second layers of wefts (20a, 20b), respectively. The first and second layers of wefts are woven by common warps (20c, 20d) in a manner that the first layer has a shorter average interweft distance than that of the second layer of wefts.In one of the conductive layers, the conductive carbon particles comprise first carbon particles and second carbon particles having larger amount of acidic function groups than that of the first carbon particles. The weight ratio of second carbon particles relative to the first carbon particles increases from one end of the gas diffusion layer corresponding to the inlet of flow channel to the other end of gas diffusion layer corresponding to the outlet of gas flow channel. The weight ratio of second polymer material relative to the first polymer material increases from one end of the gas diffusion layer corresponding to the inlet of flow channel to the other end of gas diffusion layer corresponding to the outlet of gas flow channel.Preferred Properties: Each porous supporting carbon body has water repellent property.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: Each diffusion layer comprises an electrically conductive layer comprising polymer material (preferably fluorocarbon resin) and electrically conductive carbon particles. In one of the conductive layers, the polymer material comprises first polymer material, and second polymer material having lower degree of crystallinity and higher coefficient of moisture permeability than that of the first polymer material.