NOVELTY - Carbon substrate comprises: at least two types of carbon fibers having different lengths and randomly disposed to form a non-woven fabric; and a carbide of an organic polymer disposed between the carbon fibers to bind the same. First carbon fibers having shorter lengths are predominantly disposed to be closer to the first surface of the carbon substrate in a thickness direction of the carbon substrate, and second carbon fibers having longer lengths than the first carbon fibers are predominantly disposed to be closer to the second surface of the carbon substrate.    USE - The carbon substrate is useful for a gas diffusion layer, as a porous carbon substrate with first and second surfaces opposite to each other, where the gas diffusion layer is useful in an electrode for a fuel cell (all claimed) and in a membrane-electrode assembly.    ADVANTAGE - The carbon substrate has: improved water discharge ability and improved bending strength both in the machine direction and cross-machine direction, and thus may improve performance of a fuel cell; and low resistance against mass transfer in a high current density region, consequently improving performance of a fuel cell.    DETAILED DESCRIPTION - Carbon substrate comprises: at least two types of carbon fibers having different lengths and randomly disposed to form a non-woven fabric; and a carbide of an organic polymer disposed between the carbon fibers to bind the same. First carbon fibers having shorter lengths are predominantly disposed to be closer to the first surface of the carbon substrate in a thickness direction of the carbon substrate, and second carbon fibers having longer lengths than the first carbon fibers are predominantly disposed to be closer to the second surface of the carbon substrate, so that the carbon substrate has a porosity gradient in the thickness direction of the carbon substrate that increases toward the second surface from the first surface of the carbon substrate. INDEPENDENT CLAIMS are also included for:    (1) a gas diffusion layer for a fuel cell, comprising the carbon substrate and a microporous layer disposed on the first surface of the carbon substrate and having an average pore size in the middle that is smaller than an average pore size of the carbon substrate in a region adjacent to the microporous layer; and    (2) an electrode for a fuel cell, comprising the gas diffusion layer, and a catalyst layer disposed on the microporous layer of the gas diffusion layer.