A sterilizing method for sterilizing a sterilization object housed in a chamber 11 includes a first vapor injection step S502 for injecting vapor produced from a first aqueous solution of hydrogen peroxide to an inside of the chamber 11, an ozone injection step S505 for injecting ozone gas to the inside of the chamber 11 after the first vapor injection step S502, and a second vapor injection step S507 for injecting vapor produced from a second aqueous solution of hydrogen peroxide to the inside of the chamber 11 after the ozone injection step S505. A total amount of the hydrogen peroxide included in the second aqueous solution is smaller than or equal to a total amount of the hydrogen peroxide included in the first aqueous solution.