To obtain a device effectively reducing concentration of carbon mono oxide in a hydrogen rich gas and simplifying and down sizing the device.

This carbon monooxide concentration reducing device 20, the pressure of a cooling path of a heat exchanger 33 of a first oxidizing reactor 32 is adjusted to 4-5 atmospheric pressure and the pressure of a cooling path of a heat exchanger 43 of a second oxidizing reactor 42 approximately to 2 atmospheric pressure, and the temperature of the both oxidizing reactors 32 and 42 is adjusted by an ebullient cooling of a reforming material comprising methanol and water as a hydrocarbon fuel supplied to a fuel reforming device 12 by setting them to flow in both cooling paths. The device is capable of effectively reducing concentration of carbon mono oxide in the hydrogen rich gas and down sizing the device as the result of promoting a preferential oxidation of the carbon mono oxide in the first oxidizing reactor 32 due to adjusting the temperature of the first oxidizing reactor 32 higher than the temperature of the second oxidizing reactor 42.