PURPOSE: To obtain a catalyst having superior ozone decomposing property at ≤100°C, especially about 50°C, and durability, by adding a specified proportion of NiO and Ag to cobalt oxide.

CONSTITUTION: An ozone decomposing catalyst is prepared by adding 1W30wt% NiO and 0.1W20wt% Ag based on the mole percentage of Ni and Co to cobalt oxide.(CoOx). For example, CoOx obtained by heat-decomposing basic cobalt carbonate CoCO3 Co(OH)2 in the air stream and nickel oxide obtained by heat treating nickel oxide hydroxide NiOOH in the air stream are mixed and a proper amount of silica sol is added, kneaded, and heat-treated in the air stream to give CoOx-NiO mixture. This is crushed and the obtained grains are added to an aq. soln. of silver nitrate. After immersion, they are heat-treated in the air stream to obtain an Ag-CoOx-NiO catalyst.