PURPOSE: To provide a reformer capable of being miniaturized and improving the heat efficiency.

CONSTITUTION: A concentric double inner tube 40 contg. a catalyst bed is provided in the space in a reforming tube 3, a catalyst-packed bed 4 is formed in the spaces of the inner tube and reforming tube, a cylindrical partition is furnished in the internal space of the inner tube, and a returning path 10 is formed for the generated gas so as to pass the inside and outside of the partition. A high-temp. gas passes through a heat-transfer packed bed 7, and the waste gas is discharged from a discharge passage 5. A raw gas passes through the catalyst-packed bed between the innertube 35 of the reforming tube and the inner tube 40 and then passes through the catalyst-packed bed between the the outer tube 34 of the reforming tube and the catalyst-bed inner tube, hence the raw gas is reformed, and the reformed gas passes through the returning path and then is discharged outside the reformer vessel.