Carbon sheet, gas diffusion electrode substrate and fuel cell

A gas diffusion electrode substrate comprising a microporous layer and a porous carbon sheet comprising a carbon fiber and a carbonized binding material, wherein

• the carbon sheet has a surface X1 having a surface layer area ratio X and an opposite surface Y1 having a surface layer area ratio Y, in which the surface layer area ratio X is defined as a ratio of an area of a portion of the carbon fiber and the carbonized binding material on surface X1 having a depth of 20 μm or less and the surface layer area ratio Y is defined as a ratio of an area of a portion of the carbon fiber and the carbonized binding material on the surface Y1 having a depth of 20 μm or less;
• wherein the depths are measured from a reference point of an excluded area ratio as determined by laser microscope,
• the surface layer area ratio X is larger than the surface layer area ratio Y, a difference between the surface layer area ratios is 3% or more and 12% or less, and the microporous layer is disposed on the surface X1 or a surface X2 of the carbon sheet.