A carbon monoxide transforming apparatus for fuel cell is constructed such that a catalyst having at least platinum or palladium carried on a carrier which has a base point on the surface thereof is filled in a reaction vessel having gas inlet and outlet ports. As a result, a transformation and start-up operation can be instantaneously performed on the occasion of transforming a gas containing, as main components, hydrogen, carbon monoxide, carbon dioxide and water vapor so as to convert the carbon monoxide into carbon dioxide and at the same time to generate hydrogen. Additionally, the operating temperature for the transformation can be expanded.