PURPOSE: To efficiently remove or reduce nitrogen oxide or suspended granular matter such as black smoke contained in the exhaust gas from a diesel engine by using an aqueous solution or hydrate based on a substance containing an oxy group and accelerated in its effect on an alkaline side.

CONSTITUTION: A catalyst is constituted of an aqueous solution or hydrate based on a substance (e.g. potassium carbonate) having an oxy group and accelerated in its effect on an alkaline side. By the use of this catalyst, nitrogen oxide such as nitrogen dioxide or suspended granular substance such as black smoke contained in the exhaust gas of a diesel engine of a car or the like can be efficiently removed or reduced. This catalyst can easily and inexpensively be incorporated in the engine of an existing vehicle.