NOVELTY - A carbon composite structure (15) comprises carbon nano fiber net layer; and carbon micro fiber net layer on the nano fiber layer. The carbon nano fibers of the carbon nano fiber net layer have a diameter of 100-800 nm, and pores of the layer have a pore size of 100 nm to 2.5 mu m. The carbon nano fiber net layer has a thickness of 10-100 mu m. The carbon micro fibers of the carbon micro fiber net layer have a diameter of 2-10 mu m, and pores of the layer have a pore size of 3-12 mu m. The carbon micro fiber net layer has a thickness of 100-600 mu m.    USE - As a carbon composite structure, particularly as gas diffusion layer which is used in a fuel cell (claimed).    ADVANTAGE - The two-layered carbon composite structure has a higher conductivity than that of single-layered carbon nano fiber net or the single-layered carbon micro fiber net; and has a lower pore size distribution than that of the single-layered carbon nano fiber net.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacturing carbon composite structure involving: providing an oxidized polymer nano fiber net and an oxidized micro fiber net; stacking the oxidized nano fiber net and the oxidized micro fiber net to form a stacked structure; impregnating the stacked structure in a resin; oxidizing the resin; and carbonizing the oxidized nano fiber net, the oxidized micro fiber net, and the oxidized resin, where the step of providing the polymer nano fiber net involves providing a polymer solution, and spinning the polymer solution to form polymer nano fibers that are stacked to form the polymer nano fiber net; and    (2) a fuel cell, comprising: a proton conductive film (11) disposed between two end plates (19); and a catalyst layer (13), a gas diffusion layer, a bipolar plate (17), and a current collector (18) are sequentially disposed between the proton conductive film and the end plates, respectively, where the gas diffusion layer comprises a carbon nano fiber net layer and a carbon micro fiber net layer on it to form a carbon composite structure.    DESCRIPTION OF DRAWING(S) - The figure shows a cross sectional view of a fuel cell.    Proton conductive film (11)    Catalyst layers (13)    Carbon composite structure as gas diffusion layer (15)    Bipolar plates (17)    Current collectors (18)    End plates (19)