A treatment method is capable of degrading or sterilizing harmful substances or microorganisms of which the degradation, detoxification, or the like is not easy for ozone gas alone. A harmful substance treatment method is one that makes ozone gas act on harmful substances, microorganisms, or the like in an environment humidified using a surfactant solution. The harmful substances include anticancer drugs. As a surfactant to be used, a non-ionic surfactant, an anionic surfactant, a cationic surfactant, or an amphoteric surfactant can be used.
