A fixing device is operable through start-up and warm-up stages and has a rotatable fixing member to fix an unfixed toner image borne on a recording medium, an opposing member to press against the fixing member and form a nip on the fixing member, a temperature detector to detect temperature of the fixing member, and a heater controlled in accordance with the temperature of the fixing member to heat the fixing member. The heater is further controlled during the warm-up stage in accordance with at least one of if the fixing member is rotating and if the detected temperature has ever arrived at a prescribed target temperature in the warm-up stage.