A cylindrical steam reforming unit contains a plurality of cylindrical bodies consisting of a first cylindrical body, a second cylindrical body and a third cylindrical body of successively increasing diameters disposed in concentric spaced relationship, a radiation cylinder disposed within and concentrically with the first cylindrical body, a burner disposed in the radial central portion of the radiation cylinder, and a reforming catalyst layer with a reforming catalyst filled in a gap between the first and second cylindrical bodies, wherein a CO shift catalyst layer and a CO removal catalyst layer are disposed in a gap between the second and third cylindrical bodies, the CO shift catalyst layer being formed in a gap with the direction of flow reversing at one axial end of the reforming catalyst layer and through a heat recovery layer of predetermined length. According to this reforming unit, without internally disposing a heat insulation layer, a cooling mechanism or the like, the reforming catalyst layer, CO shift catalyst layer, and CO removal catalyst layer can be integrated, achieving various useful effects, including size and weight reductions and the shortening of startup time.