The present invention provides a technique of reducing the concentration of carbon monoxide included in a hydrogen rich gas, which is fed as a supply of gaseous fuel to fuel cells. It is required that the activity of a carbon monoxide selective oxidation reaction for reducing the concentration of carbon monoxide is kept at a sufficiently high level in a wider temperature range. A CO selective oxidation unit 34, which reduces the concentration of carbon monoxide included in a hydrogen rich gas fed to fuel cells 20, has a carbon monoxide selective oxidation catalyst that includes a second element, such as lithium, in addition to ruthenium as a primary component. This arrangement enables the activity of the carbon monoxide selective oxidation reaction in the CO selective oxidation unit 34 to be kept at a sufficiently high level in a wider temperature range.