An object of the present invention is to provide a gas processing device capable of mixing together a plurality of gases sufficiently so that the gases are diluted. A gas processing device for mixing a first gas (H2) and a second gas (air) comprises a flow passage (110) through which the second gas (air) passes, an inflow port (104) for introducing the first gas (H2) in a non-parallel direction to the flow direction of the second gas (air) within the flow passage (110), and a housing (101) for sealing a part of the flow passage (110). The flow passage (110) comprises hole structures (111), through which the second gas (air) and first gas (H2) pass in and out, in at least a part of the region sealed by the housing (101). According to this constitution, the hole structures generate turbulence, and hence the degree of mixing between the first gas (H2) and second gas (air) can be improved, enabling a uniform reduction in the gas concentration.