PURPOSE: To produce a reformed gas contg. hydrogen and carbon oxides by the reaction of hydrocarbons with steam with high heat energy efficiency while stably and easily operating the reactor.

CONSTITUTION: A gaseous mixture of hydrocarbons and steam is primarily reformed by using plural heat exchanger-type reformers, and the primarily reformed gaseous reactant is secondarily reformed with an internal combustion reactor-type reformer in the presence of oxygen and/or air or by using a heat exchanger. A part of the heat to be consumed in the heat exchanger-type reformer in the primary stage is replenished with the sensible heat of the secondarily reformed gas, and the remainder is replenished with the sensible heat of the compressed air. The heat to be consumed in the secondary stage is replenished with the sensible heat of the compressed air.