To control the discharge of decomposition action substances such as chlorine which contribute to decomposition into the environment in a reaction area after the end of the decomposition treatment of a substance to be decomposed.

A polluted substance introduced into a purification tank 1 is decomposed by being irradiated with light from a light emitting means 3 in the presence of chlorine produced from a treatment solution 2. After that, an alkaline solution is supplied from an alkaline solution supply means 8 into the treatment solution 2 to reduce the amount of residual chlorine in the treatment solution.