Disclosed is a carbon fiber web including polymer nanofibers. Specifically, the carbon fiber web includes: a dispersed structure of carbon fibers; and polymer nanofibers distributed among and bonding the constituent carbon fibers of the dispersed structure. The carbon fiber web exhibits excellent characteristics in terms of flexural strength, gas permeability and electrical properties while possessing a tensile strength sufficient to undergo continuous processes for mass production. Also disclosed are a gas diffusion medium using the carbon fiber web, a gas diffusion layer including the gas diffusion medium, a membrane electrode assembly including the gas diffusion layer, and a fuel cell including the membrane electrode assembly. The use of the carbon fiber web ensures high performance of the membrane electrode assembly and the fuel cell.