A synthesis gas is produced from a carbon-containing starting material such as a coal; a CO-containing gas is separated from the resultant synthesis gas, whereby obtaining a gas containing carbon monoxide and hydrogen; a methanol- and/or dimethyl ether-containing gas is produced from the resultant gas containing carbon monoxide and hydrogen; meanwhile, a H.sub.2-containing gas is produced from the CO-containing gas and the H.sub.2O-containing gas separated from the lower-paraffin-containing gas by a shift reaction; a lower-paraffin-containing gas containing propane or butane as a main component of hydrocarbons contained therein is produced from the methanol- and/or dimethyl ether-containing gas and the H.sub.2-containing gas; and a H.sub.2O-containing gas is separated from the resultant lower-paraffin-containing gas, whereby obtaining a liquefied petroleum gas.