A fuel treatment device includes: a reforming section that produces a hydrogen-rich gas containing carbon monoxide and water; a converting section that produces a hydrogen-rich gas containing a lower concentration of carbon monoxide by reacting the carbon monoxide and the water in the hydrogen-rich gas; a mixing channel that produces a mixed gas by mixing the hydrogen-rich gas containing the lower concentration of the carbon monoxide with air containing oxygen; an air supplying section that is connected to an upstream end of the mixing channel and supplies the air to the mixing channel; and a selective oxidizing section that is connected to a downstream end of the mixing channel and converts the mixed gas into a fuel gas by reacting the carbon monoxide and the oxygen in the mixed gas, wherein the mixing channel includes a gas supply region at the upstream side and a gas diffusion region at the downstream side, and has two or more gas supply ports connecting the gas supply region with the converting section, and a length of the gas diffusion region is 0.5 to 2 times a length of the gas supply region.
