The reformer for a fuel cell system includes a reforming reaction part that generates hydrogen gas from a fuel through a catalyst reforming reaction using heat energy, and a carbon monoxide reducing part that reduces the concentration of carbon monoxide in the hydrogen gas, through an oxidizing reaction of hydrogen gas with the oxidant. The carbon monoxide reducing part includes a first reducing part including a first carbon monoxide oxidizing catalyst and a second reducing part including a second carbon monoxide oxidizing catalyst.