To provide a method for producing a gas abundantly containing hydrogen and carbon monoxide by reforming a hydrocarbon feed source with steam in the presence of a steam reforming catalyst supported as a thin membrane in a tubular reactor.

An optionally previously reformed process gas of a hydrocarbon feed source is passed and made to flow through the first tubular reactor having a thin membrane of a steam reforming catalyst supported on a wall of the reactor in the relationship with the heat conduction with a hot flue gas from the subsequent second tubular steam reforming reactor and the resultant effluent from the first tubular reactor is made to flow through the subsequent second tubular reactor heated by combustion of a fuel to thereby provide a partially steam reformed gas effluent and the hot flue gas. The effluent from the second reactor is made to flow through a fixed bed steam reforming catalyst and a product gas of a gas abundantly containing the hydrogen and carbon monoxide is recovered from the fixed bed.