To produce a catalyst for decomposing harmful substances which can efficiently decompose/remove even the high molecular weight harmful substances and a method for producing the catalyst.

In the honeycomb-shaped catalyst 10 which has a catalytic component on wall surfaces 11 and decomposes the harmful substances in gas, by setting the interval between the adjacent wall surfaces 11 to make the outer surface area (Ap value) per unit volume 400 m2/m3, even the high molecular weight harmful substances are diffused sufficiently, improving the decomposition performance of the catalyst.