To provide a reactor capable of simply and efficiently combusting both of hydrogen and fuel.

A combustor 6 is constituted by joining an upper substrate 11 and a lower substrate 12 and a micro flow passage 15 for combustion is formed on a joint surface. The micro flow passage 15 for combustion is branched, intake ports 19 to 22 are formed on one end and a discharge port 23 is formed on the other end. A combustion catalyst 34 for hydrogen is disposed on a path 31 from a combined part 30 to a combined part 32 and a combustion catalyst 35 for fuel is disposed on a mixing path 33 from the combined part 32 to the discharge port 23. Hydrogen taken-in from the hydrogen intake port 21 is mixed with air on the combined part 30 and, when flowing through the path 31, a mixed gas of hydrogen and air is combusted. Fuel taken-in from the fuel intake port 20 is mixed with air on the combined part 24 and is mixed with a product flowing from the path 31 on the combined part 32. Then, when a mixed gas comprising the fuel and air etc. flows through the path 33, the mixed gas is combusted.