In a process for producing sponge iron by direct reduction of iron oxide-containing materials, synthesis gas is mixed with the top gas produced during direct reduction of the iron oxide-containing material and used as CO- and H2-containing reduction gas for the direct reduction and heating of the iron oxide-containing materials up to a reduction temperature. In order to save energy during steel production, in particular during the refining process, one proceeds as follows during direct reduction: a carbon-containing gas, such as natural gas, or a gas with higher hydrocarbons, is used for the direct reduction besides the reduction gas; the iron oxide-containing material is exposed to the reduction gas and to the additionally supplied carbon-containing gas for a period of time longer than that required to finish reduction; and a CO/CO2-ratio from 2 to 5, preferably over 2.5, is set in the reduction gas.