Provided is a treatment method 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.
