The invention relates to a thin micro reforming apparatus used in a fuel cell system. The reforming apparatus includes a substrate having a flow path formed therein, and a fuel inlet portion for introducing fuel to the flow path of the substrate. The fuel inlet portion allows the fuel to flow through a predetermined length of a channel, thereby preheating the fuel. The reforming apparatus also includes an evaporator having a flow path for gasifying liquid fuel, and a reformer for reforming fuel into hydrogen gas via heat absorption reaction. The reforming apparatus further includes a CO remover for removing CO gas contained in the hydrogen gas via heat generation reaction, and a cover for sealing the flow paths of the substrate. The channel of the fuel inlet portion surrounds the evaporator, reformer and CO remover, thereby facilitating insulation at the sides of the reforming apparatus and increasing heat efficiency.