The present invention enables efficient decomposition work without involving burdensome operations of carrying out decomposition of undecomposed pollutants discharged at the time of starting decomposition processing and undecomposed pollutants remaining at the time of interruption and termination of decomposition processing, separately from primary decomposition. At the time of starting decomposition, the steps of supplying a substance having a function to decompose the pollutant to a decomposition area, irradiating the decomposition area with light and supplying a decomposition target substance to the reaction area are carried out in the described order, while at the time of ending start of decomposition, the operations of supplying the decomposition target substance, irradiating the decomposition area with light and supplying the substance having a function to decompose the pollutant to the decomposition area are carried out in the described order.