Plastic containers fed by an article feeder housed in a shield chamber are sterilized by being irradiated with an electron beam that is radiated from an irradiation window of an electron beam irradiation unit which is coupled to an opening of the shield chamber. In a maintenance mode, an adjustment irradiation box is detachably mounted on the electron beam irradiation unit in covering relation to the irradiation window of the electron beam irradiation unit. The adjustment irradiation box houses therein an electron beam receiver for receiving the electron beam radiated from the irradiation window, a cooling mechanism for cooling the electron beam receiver, and an exhaust mechanism for discharging an atmosphere in the adjustment irradiation box.