There has been a demand for providing a porous electrode substrate having reduced bulk density, having good handling properties and gas permeability, which has conventionally been difficult to produce. A porous electrode substrate having a structure wherein carbon fibers (A) are joined by carbides, the porous electrode substrate having a bulk density of 1.0-2.0 × 10 -1 g/cm 3 or less when a pressure of 1.5 × 10 -1 MPa is applied, and having a bending strength of at least 10 MPa.