A method of warming a fixing device includes the steps of: providing a first fixing member and a second fixing member for fixing a toner image onto a sheet, a temperature increase rate of the first fixing member being higher than that of the second fixing member; warming up the first fixing member until a first temperature when humidity is low based on humidity information; and performing an additional warm-up of the fixing device for a predetermined time after warming up the first fixing member until the first temperature, when the humidity is high.