10 ppm the internal pressure of the device is largely increased examples no 05 and no 06 represent cases where the volume of the first catalyst is 50 cc and 65 cc and smaller than 80 cc respectively these examples nos 05 and 06 are not self ignited one reason why the examples nos 05 and 06 are not self ignited is that the volume of the first catalyst is smaller than approximately 80 cc as described above in the prox device 50 a of the embodiment the volume of the first catalyst 63 is smaller than the volume of the second catalyst 83 moreover it is preferable that the volume of the first catalyst 63 is equal to or larger than 80 cc further the volume of the first catalyst 63 is approximately 0 004 times the flow rate per unit time minute of the hydrogen containing gas supplied to the first internal space 61 through the first opening portion 62 this value is the minimum volume for self ignition by the first catalyst 63 of the hydrogen containing gas passing through the first catalyst 63 fig4 is a schematic cross sectional view showing a preferential oxidation reactor according to a comparative example the prox device of the comparative example is used to represent characteristics of the prox device of the embodiment more clearly except that the separation layer and the second catalyst are not provided the prox device of the comparative example has substantially the same structure and shape as the prox device of fig3 further the prox device of the comparative device may correspond to example no 02 of table 1 referring to fig4 the prox device 50 b of the comparative example has a granule type catalyst 63 charged in the internal space 52 of.