Provided is a carbon substrate for a gas diffusion layer of a fuel cell. The carbon substrate has a structure, in which a plurality of unit carbon substrates are stacked. Each of the unit carbon substrates is a plate type substrate having a first surface and a second surface opposite to the first surface. Carbon fibers are randomly arranged on the first surface of the each unit carbon substrate. The number of the carbon fibers arranged in a machine direction of the unit carbon substrate is greater than the number of carbon fibers arranged in a transverse direction of the unit carbon substrate from the first surface to the second surface along a thickness direction of the unit carbon substrate; and, accordingly, an orientation gradient, in which the orientation in the machine direction increases from the first surface to the second surface, is shown.