A fixing device includes: a fixing-surface-side member disposed on a fixing surface side on which a toner image is to be formed in a recording medium; a back-surface-side support member that is disposed on an opposite surface side to the fixing surface in the recording medium, and forms a fixing nip that nips and conveys the recording medium with the fixing-surface-side member; a heat source that heats the toner image formed on the recording medium passing through the fixing nip; a nip time changing part that can change a nip time that is a time required for the recording medium to pass through the fixing nip; and a hardware processor that controls the nip time changing part and the heat source to shorten the nip time and increase a fixing temperature of the toner image, and to lengthen the nip time and lower a fixing temperature of the toner image.