The fixing unit satisfies following three conditions. That is, (1) 2.4103d/(TCt)T0,
where d is a thickness of the unfixed toner layer in meters, TC is a thermal conductivity
of toner in W/mK, and t is the fixing time in seconds; (2) a temperature Ttop
of a topmost layer of the toner layer is not greater than the minimum temperature
TOFF at which the hot offset of the surface of the first fixing member
occurs when the fixing time is set to 1 seconds; and (3) a temperature Tbot
of a bottommost layer of the toner layer that is in contact with the recording
medium is not less than the lower limit temperature TMIN for fixing
of the surface of the first fixing member when the fixing time is set to 1 second.
