To obtain a lightweight porous carbonaceous substrate having isotropic electric conductivity and an increased dielectric tangent by heating a synthetic resin in a nonoxidizing atmosphere.

A filler selected from among graphite a metal oxide (salt), glass fibers, graphite fibers and amorphous carbon and a dehydrating and foam stabilizer made of boric acid (salt) are added to 100 pts.wt. synthetic resin by 40-370 pts.wt. and 0.01-15 pts.wt., respectively, and they are blended and heated to the decomposition temp. of the resin or above in a nonoxidizing atmosphere to obtain the objective porous carbonaceous substrate having ≥85% carbon content, 0.5-3mm average pore diameter, 0.2-1.5g/cm3 density, 50-200kg/cm2 bending strength, a dielectric constant of 2-8 and a dielectric tangent of 10-10-10-3. In the substrate, the atomic ratio of C:H is 1:(0.8-1.5).