The removal of photoresist with novolac resin on a semiconductor wafer by high concentration aqueous ozone was found to proceed by a two-stage chemical process. It was also found that the two-stage removal process of the photoresist resin is accelerated by the optimized ultraviolet rays. That is, in the first stage, hydroxide radicals produced from the ozone by the ultraviolet rays oxidize the photoresist resin into the polyphenol. In the second stage, the polyphenol is easily removed from the semiconductor wafer by the ozone, since the polyphenol is more reactive to the ozone, which acts to remove electrons from the polyphenol. On the other hand, since too strong ultraviolet ray change most of the ozone into oxygen, removal efficiency is controlled by the balance between the ozone concentration and the intensity of ultraviolet rays.