PROBLEM TO BE SOLVED: To provide a porous electrode substrate precursor sheet which is low in production cost in producing a precursor sheet and reduced in fiber bundles in which carbon fibers are bundled, and to provide a method for producing the same.SOLUTION: There is provided a method for producing a porous electrode substrate precursor sheet by making paper from a fiber mixed slurry composed of a mixture containing carbon short fibers (A) and precursor fibers (b), in which the following (I) to (IV) are performed. (I) A dewaterable adjusting agent is mixed with a dispersion medium (A) to obtain dispersion medium (B) containing a dewaterable adjusting agent. (II) The dispersion medium (B) obtained in (I) and the carbon short fibers (A) are mixed to obtain a slurry (A). (III) The slurry (A) obtained in (II) and the precursor fibers (b) are mixed to obtain a slurry (B). (IV) The slurry (B) obtained in (II) is made into paper to obtain a porous electrode substrate precursor sheet composed of a fiber mixture o
