This invention relates to an oxidative autothermal reformer (1) comprising a reforming layer (2) at least partially filled with a reforming catalyst for producing a reformed gas composed mainly of hydrogen through a reforming reaction by contacting a mixture of a hydrocarbon or an aliphatic alcohol and steam with the reforming catalyst; and an oxidative exothermic layer (3) at least partially filled with an oxidation catalyst for generating heat by oxidizing a part of the reformed gas, in which the reforming layer (2) is disposed at an upstream side of the oxidative exothermic layer (3); the reforming layer (2) and the oxidative exothermic layer (3) are in the form of a cylinder and have a triple circular tube structure formed by disposing an inner reforming layer (2A), an oxidative exothermic layer (3) and an outer reforming layer (2B) from the inside in a radical direction in this order; and at least a part of the reforming catalyst filled in the inner reforming layer (2A) and the outer reforming layer (2B) contains Ru metal.