a substrate treating method for cleaning a substrate by supplying a cleaning solution thereto . the method comprises the steps of supplying the cleaning solution having ozone dissolved therein to the substrate , and irradiating the cleaning solution with ultraviolet light . by irradiating the cleaning solution having ozone dissolved therein with ultraviolet light , oxygen radicals are generated easily to increase the activity of the cleaning solution . thus , a significantly improved cleaning capability is achieved even with low concentration ozone water . this method is applicable also to a piecemeal treatment for cleaning large substrates . since the cleaning solution supplied to the substrate contains ozone in a low concentration , a filter and piping materials for supplying the cleaning solution need not have strong ozone resistance .