PURPOSE: To convert nitrogen oxides (NO, NO2, etc.) discharged from automobiles and burners such as home burners and various kinds of industrial burners, into harmless N2, O2 without using reducing agent by passing through a filter.

CONSTITUTION: The contaminated air containing NOx is introduced from a suction opening 1 and then flowed through NOx decomposition catalyst 2 in which NOx is decomposed into N2 and O2. Slight amounts of CO and various kinds of hydrocarbons contained in the decomposed gas are absorbed by an activated carbon layer 3, and the clean air is discharged from a discharge opening 4. The catalyst 2 is constituted by carrying the compound oxides of AxByCzOα-β crystal structural formula with 0W1% platinum group metal such as Pt and Pd on the surface of inorganic heat-resisting material, such as alumina and silica, where A, B and C are respectively selected from alkaline earth metals such as Ba and Sr, rare earth elements such as Y, Nd and La, and copper group metals such as Cu and Ag, and suffixes (x), (y), (z), α, β are selected from the range as follows: 0≤x≤3.0, 0≤y≤2.0, 0≤z≤4.0, 4.0≤α≤8.0, 0≤β≤2.0.