A reformer of a fuel cell system is disclosed. One embodiment of the reformer includes a reforming reactor generating reformed gas containing hydrogen by reforming hydrogen-containing fuel; and a CO remover removing carbon monoxide contained in the reformed gas generated from the reforming reactor, wherein the CO remover is disposed to be inclined in a predetermined angle to a moving path of the reformed gas exhausted from the reforming reactor and connected to the reforming reactor so as to communicate fluid therebetween, whereby the CO remover is not subject to the heat energy effect by a heat transfer effect due to air convection from the reforming reactor, making it possible to keep the CO remover an optimal state to improve reforming efficiency.