The invention relates to a thin micro reforming apparatus used in a fuel cell system with decreased back pressure in an evaporator thereof. In the reforming apparatus, a substrate has a flow path therein and a fuel inlet portion for introducing fuel into the flow path of the substrate. An evaporator includes a gas expansion part which gasifies the liquid fuel and a back pressure accommodation part which accommodates back pressure during the gasification to block the influence of the back pressure to the fuel inlet portion. A reformer has a flow path formed downstream of the evaporator, reforming fuel into hydrogen gas via heat absorption reaction. A CO remover has a flow path formed downstream of the reformer, removing CO gas contained in the hydrogen gas via heat generation reaction. A cover is attached to an upper part of the substrate to seal the substrate from the external environment.