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.