A method for treating an inner wall surface of a treatment object uses a treatment object that is at least one of a container housing an ozone gas, a treatment container housing an object to be subjected to a surface treatment using an ozone gas and a pipe configured to supply an ozone gas. The method for treating an inner wall surface of a treatment object includes the steps of: determining whether an abnormal part is present in the inner wall surface of the treatment object or not; and distributing an ozone gas having a concentration of 10% by volume or more and 30% by volume or less and a temperature of 60° C. or less such that the ozone gas contacts the inner wall surface of the treatment object after the step of determining whether an abnormal part is present or not.