The present invention provides a porous electrode substrate that has low production cost, high mechanical strength, thickness precision, and surface smoothness, and sufficient gas permeability and electrical conductivity, and a method for producing the same. In the present invention, for example, a porous electrode substrate that includes short carbon fibers (A) joined together via three-dimensional mesh-like carbon fibers (B) is produced by a method including a step (1) of dispersing short carbon fibers (A), and short carbon fiber precursors (b) to be fibrillated by beating, to produce a precursor sheets; and a step (2) of subjecting the precursor sheet to carbonization treatment at a temperature of 1000°C or higher.