To dissolve and peel inorganic scale attached to a storage tank storing water to be treated, pipes and jackets of ultraviolet lamps without using any chemical.

This photocatalytic water treatment apparatus decomposes organic and inorganic substances in water 100 to be treated using the storage tank 11, and is provided with an apparatus body part 2 disposed outside the storage tank 11; a circulating line consisting of a water taking-in pipe 8 connected to a water taking-in side of the body 2 and a discharge pipe 9 connected to a discharge side of the body 2; a pump 4 circulating the water 100 to be treated; a filter 5 provided in the body 2; a photocatalytic treatment means provided on a downstream side of the filter 5 using photocatalyst carriers 7 and the ultraviolet lamps 6; and an electrode part 3 provided in the tank 11. The water 100 to be treated is electrolyzed by energizing the electrode part 3.