A hydrocarbon gas to be decomposed, optionally mixed with water vapor, is allowed to flow through the gap between various porous tubes 4 constituting a group of porous tubes provided extending vertically in a decomposition reaction column 1 while air or oxygen is jetted homogeneously from the interior to the exterior of the porous tubes in the direction perpendicular to the stream of the hydrocarbon gas to be decomposed, optionally mixed with water vapor. The mixture is then ignited to form a diffusion flame layer B on the outer surface of the porous tubes 4. With the diffusion flame layer as a heat source, the hydrocarbon gas to be decomposed, optionally mixed with water vapor, undergoes pyrolysis.