A reformer of the present disclosure includes a reformer body having a cylindrical shape that carries out a reforming reaction by a raw fuel gas and water supplied thereto, the reformer body including therein a vaporization portion which generates steam and a reforming portion which reacts the steam generated in the vaporization portion with the raw fuel gas to generate a reformed gas, at least one of a convex portion and a rough portion having a higher degree of surface roughness than that of other portions, being disposed on at least one of an inner circumferential surface and an outer circumferential surface of the reformer body.