A dinitrogen pentoxide generating device and a method for generating dinitrogen pentoxide. An NOx generating unit is able to use, as a raw material gas, a gas containing nitrogen and oxygen and produce a plasma to generate a nitrogen oxide. An ozone generating unit is able to use, as a raw material gas, the gas containing nitrogen and oxygen or a gas obtained after the plasma has been produced by the NOx generating unit and produce a plasma to generate ozone. A mixing unit is able to hold the nitrogen oxide generated by the NOx generating unit and the ozone generated by the ozone generating unit in the same space for a predetermined time to generate dinitrogen pentoxide.