To make it possible to decompose the nitrogen monoxide from a discharge source to nitrogen and oxygen with high efficiency regardless of a concn. region and without requiring a reducing agent by holding aluminum polynuclear oxide cation between the layers of montmorillonite which is a swellable clay mineral having a laminar structure.

The nitrogen monoxide decomposing material capable of decomposing the nitrogen monoxide to nitrogen and oxygen is prepd. from the material formed by holding the aluminum polynuclear oxide cation 3 between the layers of the montmorillonite 1 which is the swellable clay mineral. At this time, the inter-layer spacings of the montmorillonite 1 are specified to 1.5 to 2.0nm. The oxide cation expressed by [Al13(OH)24(H2O)12]7+ is used as aluminum polynuclear oxide cation 3. Further preferably, tetra(1-methyl-4-pyridyl}porphyrin, etc., are held between the layers of the montmorillonite 1. Copper ions 5 are otherwise held therebween and, further, the aluminum polynuclear oxide cation 3 is held therebetween.