An insoluble and infusible substrate with a polyacene-type skeletal structure having a hydrogen/carbon atomic ratio of from 0.05 to 0.60 and a specific surface area, measured by the BET method, of at least 600 m.sup.2 /g, said substrate being a heat-treated product of a mixture comprising an aromatic condensation polymer consisting of carbon, hydrogen and oxygen as a matrix and a fiber member selected from the group consisting of phenol resin fibers, heat-treated phenol resin fibers and carbon fibers dispersed therein.The substrate has excellent physical properties such as strength and is useful as an electrode for organic cell.