A situation where sulfur compounds originating from a castable are mixed into synthesis gas produced by way of a reforming reaction and the mixed sulfur compounds are separated and collected with carbon dioxide and further fed into a reformer to thereby degrade the reforming catalyst of the reformer by sulfur poisoning is avoided. Purge gas that is steam or steam-containing gas is made to flow into the piping to be used for a synthesis gas production apparatus and dried out to remove the sulfur compounds contained in the castable prior to the start-up of operation of the synthesis gas production apparatus, in order to prevent the sulfur compounds from being released by hot synthesis gas.