To prevent the deterioration of catalytic activity of a NOx occluding and reducing catalyst or each of other various catalysts by the potassium compo nent contained in these catalysts.

This method is carried out by immersing the potassium- containing catalyst in the aqueous solution of L (+)-tartaric acid, D (+)-tartaric acid or DL-tartaric acid so that the potassium in the catalyst is fixed as potassium bitartrate. The potassium-containing catalyst is manufactured by fixing the potassium component by this method.