Abstract: A dielectric or induction heating fixing device utilizing a high-frequency wave such as microwave for fixing an unfixed image on an image bearing member stabilizes the fixing effect in accordance with the size of the image bearing member. A control device controls the applied high-frequency wave in accordance with the size of the image bearing member, a conveyer conveys one side edge of the image bearing member of each size along a conveyance standard or reference, and a device regulates the area to which the high-frequency wave is applied, in accordance with each size. In a preferred embodiment, there is provided a control device for controlling the applied high-frequency wave so that the fixativeness of the unfixed image after the image bearing member has passed the area to which the high-frequency wave is applied becomes equal irrespective of the size of the image bearing member.