A sulfur compound derived from a castable is mixed into a synthesis gas produced by a reforming reaction, the mixed sulfur compound is separated and collected along with carbon dioxide gas, and the collected sulfur compound is supplied as is into a reformer, thereby avoiding the degradation of a reforming catalyst in the reformer from sulfur poisoning. Before operating a synthesis gas production device, a purge gas comprising steam or a steam-containing gas is flushed through pipes used in the synthesis gas production device and dried out through firing, and the sulfur compound included in the castable is removed in advance, thereby preventing desorption of the sulfur compound by the high temperature synthesis gas.