A diffusion membrane contains uncalcined polytetrafluoroethylene (PTFE), calcined PTFE and a conductive substance. An electrode has the diffusion layer (membrane) and a catalyst layer disposed thereon. A process for producing diffusion membranes comprises dispersing uncalcined PTFE, calcined PTFE and a conductive substance in a dispersion medium, removing the dispersion medium, adding a liquid lubricant followed by kneading, compressing the resultant kneaded product (preferably a rod-shaped preform) to expand the same into a membrane, and removing the liquid lubricant. The diffusion membrane preferably contains 1 to 30 wt % of the uncalcined PTFE and 5 to 30 wt % of the calcined PTFE. The conductive substance is desirably a combination of graphite and carbon black. The diffusion membrane is free from local variation in properties, has suitable mass producibility and can be produced inexpensively.