To provide a photocatalyst which enables quickly decomposing and removing photocatalytically reactive substances such as a malodorous substance in the air, a plant growth promoter and a poisonous substance and is resistant to shock and vibration.

This photocatalyst comprises a hydration aggregate 1 of a formed clay or a hydraulic compound which contains semiconductors having band gaps of 0.5-5 eV and an electrically conductive wire material 2. This usage of the photocatalyst is composed of a process of heating the photocatalyst, a process of bringing the gas including photocatalytically reactive substances into contact with the photocatalyst and a process of irradiating the photocatalyst with ultraviolet rays.