PURPOSE: To effectively decompose a malodor by the action of a photocatalyst by supporting a semiconductor on a cubic porous body, passing gas contg. the malodor through the resulting photocatalyst and irradiating the semiconductor with light.

CONSTITUTION: A cubic porous body of ceramics, etc., produced from a fibrous structure as starting material is dipped in a titania sol to impregnate the sol into the porous body and this porous body is dried and heat-treated to support titanium oxide to obtain a photocatalyst 1. The resulting photocatalyst 1 is set on the bottom of a stainless reactor 2 so that the photocatalyst confronts a window 3 of a quartz sheet and a UV lamp 4 is laid just above the window 3. Gas contg. a malodor and air are allowed to flow in the reactor 2 and the lamp 4 is put on to irradiate the photocatalyst 1 with light. The gas comes in very satisfactory contact with the photocatalyst and the decomposition reaction of the malodor with the photocatalyst proceeds efficiently without shielding UV.