Disclosed is a method for photolyzing organic matters by which various organic matters including harmful organic matters can be photolyzed quickly. Also disclosed is an efficient method for treating wastewater which utilizes the photolyzing method. An organic matter is photolyzed in a reaction system containing a photocatalyst and a hydrophobic ionic liquid represented by the formula (2) below. ##STR00001## [In the formula, Me represents a methyl group and Et represents an ethyl group.]