New porous materials with hybrid pore structure “Fibers-In-Pores” structure were produced by developing Si3N4 whiskers in pores of partially-sintered silicon by nitriding. Because silicon and Si3N4 have excellent heat resistance, the proposed porous materials are expected to utilize high-temperature applications such as gas filters. Porous silicon was produced by sintering Si powder at 1300°C for 1 h in a stream of Ar gas. Porous Si bodies were nitrided at 1300°C for 1 h with N2 gas to develop long Si3N4 whiskers in pores of partially-sintered silicon. Specific surface area was increased and gas permeability was decreased by applying the nitridation process at 1300°C.