A gas diffusion electrode substrate has an electrically conductive porous substrate and a microporous layer-1 on one side of the electrically conductive porous substrate. The microporous layer-1 includes a dense portion A and a dense portion B. The dense portion A is a region containing a fluorine resin and a carbonaceous powder having a primary particle size of 20 nm to 39 nm. The dense portion A has a thickness of 30% to 100% with respect to the thickness of the microporous layer-1 as 100% and a width of 10 μm to 200 μm. The dense portion B is a region containing a fluorine resin and a carbonaceous powder having a primary particle size of 40 nm to 70 nm.