NOVELTY - A fuel-treating apparatus has a modifier generating hydrogen rich gas containing carbon monoxide and water, a transformation unit, a mixing flow path, an air supply portion and a selective-oxidation portion connected to the downstream end of mixing flow path. The mixing flow path comprises an upstream gas supply area and downstream gas diffusion area, and has 2 or more gas supply ports connecting gas supply area and transformation unit. The length of gas diffusion area is 0.5-2 times the length of gas supply area.    USE - Apparatus is used for producing fuel gas used for fuel cell system e.g. domestic co-generation system.    ADVANTAGE - The fuel-treating apparatus stably produces fuel gas having low carbon monoxide concentration with reduced pressure loss. The carbon monoxide in hydrogen rich gas is efficiently oxidized in the selective-oxidation portion of fuel-treating apparatus.    DETAILED DESCRIPTION - A fuel-treating apparatus has a modifier generating hydrogen rich gas containing carbon monoxide and water, a transformation unit, a mixing flow path, an air supply portion and a selective-oxidation portion connected to the downstream end of mixing flow path. The carbon monoxide and water in hydrogen rich gas are reacted in the transformation unit, and hydrogen rich gas having low carbon monoxide concentration is produced. The hydrogen rich gas having low carbon monoxide concentration is mixed with air containing oxygen in the mixing flow path, and mixed gas is generated. The air supply portion is connected with the upper end of the mixing flow path, and supplies air to the mixing flow path. The carbon monoxide and oxygen in the mixed gas are reacted in the selective-oxidation portion, and fuel gas is generated. The mixing flow path comprises an upstream gas supply area and downstream gas diffusion area, and has 2 or more gas supply ports connecting gas supply area and transformation unit. The length of gas diffusion area is 0.5-2 times the length of gas supply area.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of mixing flow path.    Mixing flow path (100)    Air inflow port (101)    Gas supply region (110)    Gas supply ports (111,112)    Gas diffusion region (120)