To provide a hydrogen-containing gas producing apparatus which can be smoothly operated by improving the activity of a carbon monoxide removing catalyst.

In the method for operating the hydrogen-containing gas producing apparatus provided with at least a reformer 3 for producing the hydrogen- containing gas containing hydrogen by reforming raw materials and fuel, a transformer 4 for transforming carbon monoxide contained in the hydrogen- containing gas to carbon dioxide with a carbon monoxide transforming catalyst, and a carbon monoxide remover 5 for decreasing the concentration of carbon monoxide contained in the hydrogen-containing gas supplied from the transformer 4 after being transformed to equal to or below a prescribed value by using a carbon monoxide removing catalyst 6, a catalyst drying process for drying the carbon monoxide removing catalyst 6 before the transformed hydrogen-containing gas is supplied from the transformer 4 to the carbon monoxide remover 5 in the start-up of the hydrogen-containing gas producing apparatus is included.