A sterilizing system including a sterilizing device configured to generate plasma and to effect sterilization of a sterilization-subject article with a sterilization agent containing the generated plasma, and a chamber inside which the sterilizing device effects the sterilization; wherein: (a) the sterilizing device includes: - a plasma-producing unit; - a gas supply unit and oxygen supply passage to supply oxygen to the plasma-producing unit for the production of oxygen plasma containing ozone; - an outlet, and a relay section between the plasma-producing unit and the outlet, and - an evaporator connected to a steam supply passage to generate and feed steam to the relay section whereby in use reactive oxygen is produced by reaction between the supplied steam and the oxygen plasma, and the oxygen plasma containing ozone and the reactive oxygen are discharged together from the outlet as the sterilizing agent; and (b) the chamber has an ozone collecting unit configured to collect ozone contained in the discharged oxygen plasma and return this ozone to the sterilizing device by a conduction passage, the conduction passage connecting into the oxygen supply passage, or to the plasma-producing unit directly, for supply of the collected ozone to the relay section along with the oxygen plasma containing ozone, for the reaction with the steam, wherein the system further includes: an ozone meter for determining an amount of ozone collected by the ozone collecting unit; and an oxygen supply control unit for adjusting the amount of oxygen to be supplied to the sterilizing device based on the amount of ozone determined by the ozone meter. a sterilizing device configured to generate plasma and to effect sterilization of a sterilization-subject article with a sterilization agent containing the generated plasma, and a chamber inside which the sterilizing device effects the sterilization; wherein: (a) the sterilizing device includes: - a plasma-producing unit; - a gas supply unit and oxygen supply passage to supply oxygen to the plasma-producing unit for the production of oxygen plasma containing ozone; - an outlet, and a relay section between the plasma-producing unit and the outlet, and - an evaporator connected to a steam supply passage to generate and feed steam to the relay section whereby in use reactive oxygen is produced by reaction between the supplied steam and the oxygen plasma, and the oxygen plasma containing ozone and the reactive oxygen are discharged together from the outlet as the sterilizing agent; and (b) the chamber has an ozone collecting unit configured to collect ozone contained in the discharged oxygen plasma and return this ozone to the sterilizing device by a conduction passage, the conduction passage connecting into the oxygen supply passage, or to the plasma-producing unit directly, for supply of the collected ozone to the relay section along with the oxygen plasma containing ozone, for the reaction with the steam, wherein the system further includes: an ozone meter for determining an amount of ozone collected by the ozone collecting unit; and an oxygen supply control unit for adjusting the amount of oxygen to be supplied to the sterilizing device based on the amount of ozone determined by the ozone meter. (a) the sterilizing device includes: - a plasma-producing unit; - a gas supply unit and oxygen supply passage to supply oxygen to the plasma-producing unit for the production of oxygen plasma containing ozone; - an outlet, and a relay section between the plasma-producing unit and the outlet, and - an evaporator connected to a steam supply passage to generate and feed steam to the relay section whereby in use reactive oxygen is produced by reaction between the supplied steam and the oxygen plasma, and the oxygen plasma containing ozone and the reactive oxygen are discharged together from the outlet as the sterilizing agent; and - a plasma-producing unit; - a gas supply unit and oxygen supply passage to supply oxygen to the plasma-producing unit for the production of oxygen plasma containing ozone; - an outlet, and a relay section between the plasma-producing unit and the outlet, and - an evaporator connected to a steam supply passage to generate and feed steam to the relay section whereby in use reactive oxygen is produced by reaction between the supplied steam and the oxygen plasma, and the oxygen plasma containing ozone and the reactive oxygen are discharged together from the outlet as the sterilizing agent; and (b) the chamber has an ozone collecting unit configured to collect ozone contained in the discharged oxygen plasma and return this ozone to the sterilizing device by a conduction passage, the conduction passage connecting into the oxygen supply passage, or to the plasma-producing unit directly, for supply of the collected ozone to the relay section along with the oxygen plasma containing ozone, for the reaction with the steam, wherein the system further includes: an ozone meter for determining an amount of ozone collected by the ozone collecting unit; and an oxygen supply control unit for adjusting the amount of oxygen to be supplied to the sterilizing device based on the amount of ozone determined by the ozone meter. an ozone meter for determining an amount of ozone collected by the ozone collecting unit; and an oxygen supply control unit for adjusting the amount of oxygen to be supplied to the sterilizing device based on the amount of ozone determined by the ozone meter.