Apparatus for heating plastic preforms, having a microwave generating device which generates microwaves, and having a resonator device which forms a receiving space into which the plastic preforms can be introduced so as to be acted upon by the microwaves in the resonator device in order to be heated, wherein the apparatus has a reflector element which is arranged at least partially in the receiving space and which is suitable and intended for conducting microwaves in the direction of a base region of the plastic preforms. According to the invention, the reflector element is movable relative to the receiving space and the apparatus has a drive device for moving the reflector element relative to the receiving space.