A burner combustion chamber (3), a burner (7) for performing a burner combustion in the burner combustion chamber (3) a reformer catalyst (4) to which burner combustion gas is fed, and a heat exchange part (13a) for heating the air fed to the burner (7) are provided. A switching device (16, 17) able to switch an air flow route for introducing the outside air to the burner (7) between a high temperature air flow route (13) for introducing the outside air flowing within the heat exchange part (13a) and heated at the heat exchange part(13a) to the burner (7) and a low temperature air flow route (14) for feeding the outside air, which does not flow within the heat exchange part (13a) and thereby is lower in temperature than the outside air heated at the heat exchange part (13a), to the burner (7) is provided.