The invention discloses an apparatus (1) for heating a plastic parison (10), comprising: a microwave generating means (2) for generating microwaves; and a resonator means (6) forming an accommodation space (62) wherein the parison (10) can be introduced into the accommodating space (62) so as to be heated by microwave action in the resonator device (6); wherein the device (1) has a reflector element (8); the element (8) is at least partially disposed in the accommodation space (62), and the reflector element (8) is adapted to conduct microwaves in the direction of the bottom (10a) of the plastic parison (10). According to the invention, the reflector element (8) is movable relative to the holding space (62), and the apparatus (1) has a driving device (64) which moves the reflector element (8) relative to the holding space (62).