A synthesis gas production apparatus (reformer) to be used for a synthesis gas production step in a GTL (gas-to-liquid) process is prevented from being contaminated by metal components. A method of suppressing metal contamination of a synthesis gas production apparatus operating for a GTL process that includes a synthesis gas production step of producing synthesis gas by causing natural gas and gas containing steam and/or carbon dioxide to react with each other for reforming in a synthesis gas production apparatus in which, at the time of separating and collecting a carbon dioxide contained in the synthesis gas produced in the synthesis gas production step and recycling the separated and collected carbon dioxide as source gas for the reforming reaction in the synthesis gas production step, a nickel concentration in the recycled carbon dioxide is not higher than 0.05 ppmv.