An apparatus and a method of sterilizing inner walls of containers, wherein the apparatus has at least one electron beam emitter with at least one electron beam accelerator and an outlet window for the electron beams, a conveying device for conveying the containers to be sterilized and a reciprocating device for permitting a relative movement between the containers and the outlet window in a longitudinal direction of the containers. A reflector apparatus is connected to the electron beam emitter in a positively locking and/or friction locking manner at least locally in a region of the outlet window and is capable of being introduced at least locally during a defined period of time into an interior space of the container to be sterilized, in order to apply the electron beams to the inner walls of the container.