A fixing device includes a driving unit, a pressing roller, a heater, a fixing belt, a pressing roller, a nip formation pad that presses against an inner surface of the fixing belt toward the pressing roller via the fixing belt to form a nip, a separation mechanism, a lubricant between the fixing belt and the nip formation pad, and a processor to execute a warming-up operation of the fixing device. In the warming-up operation, before the heater heats the fixing belt, the separation mechanism moves the nip formation pad in a direction of separating the nip formation pad from the pressing roller to set a nip pressure lower than an ordinary nip pressure, and the driving unit rotates the pressing roller that rotates the fixing belt. Thereafter, the heater starts heating, and the separation mechanism moves the nip formation pad to contact the fixing belt at an ordinary nip pressure.