A method for maintaining sterility in an electron beam irradiation treatment of an irradiation target using a sterilization electron beam irradiation apparatus to which said irradiation target is conveyed consecutively for irradiation, the apparatus comprising: a pre-pressure adjusting bath; a post-pressure adjusting bath; an irradiation treatment bath having a pressure reducing means that maintains internal pressure thereof at negative state; and a rotary carrier (11, 21, 31) arranged rotatively in each of said baths (10, 20, 30), wherein said pre-pressure adjusting bath and said post-pressure adjusting bath are integrally jointed to the side face portion of said irradiation treatment bath, an auxiliary electron beam generating means is provided at the joint between said irradiation treatment bath and said pre-pressure adjusting bath for sterilizing gas by irradiation with electron beam; and a clean air feeding means is connected with each of said pre-pressure adjusting bath and said post-pressure adjusting bath severally through an air duct. said method comprising the steps of: flowing clean air from said post-pressure adjusting bath side to said pre-pressure adjusting bath side and discharging said clean air from said pre-pressure adjusting bath; pressurizing said post-pressure adjusting bath at its adjoining side to said irradiation treatment bath to a pressure higher than the pressure at an air discharging port on said pre-pressure adjusting bath; and sterilizing gas existing in the joint between said irradiation treatment bath and said pre-pressure adjusting bath by irradiating said gas with said auxiliary electron beam. a pre-pressure adjusting bath; a post-pressure adjusting bath; an irradiation treatment bath having a pressure reducing means that maintains internal pressure thereof at negative state; and a rotary carrier (11, 21, 31) arranged rotatively in each of said baths (10, 20, 30), wherein said pre-pressure adjusting bath and said post-pressure adjusting bath are integrally jointed to the side face portion of said irradiation treatment bath, an auxiliary electron beam generating means is provided at the joint between said irradiation treatment bath and said pre-pressure adjusting bath for sterilizing gas by irradiation with electron beam; and a clean air feeding means is connected with each of said pre-pressure adjusting bath and said post-pressure adjusting bath severally through an air duct. said method comprising the steps of: flowing clean air from said post-pressure adjusting bath side to said pre-pressure adjusting bath side and discharging said clean air from said pre-pressure adjusting bath; pressurizing said post-pressure adjusting bath at its adjoining side to said irradiation treatment bath to a pressure higher than the pressure at an air discharging port on said pre-pressure adjusting bath; and sterilizing gas existing in the joint between said irradiation treatment bath and said pre-pressure adjusting bath by irradiating said gas with said auxiliary electron beam. flowing clean air from said post-pressure adjusting bath side to said pre-pressure adjusting bath side and discharging said clean air from said pre-pressure adjusting bath; pressurizing said post-pressure adjusting bath at its adjoining side to said irradiation treatment bath to a pressure higher than the pressure at an air discharging port on said pre-pressure adjusting bath; and sterilizing gas existing in the joint between said irradiation treatment bath and said pre-pressure adjusting bath by irradiating said gas with said auxiliary electron beam.