A developing unit is disclosed that includes a sub rotary developing unit,
a toner supply device, and a drive device. The sub rotary developing unit
has a developing roller in the interior thereof, and an elastic seal made
of an elastic member with a slit sealing an opening on the sub rotary
developing unit. The toner supply device supplies toner from a nozzle at
the tip thereof to the sub rotary developing unit. The drive device
drives the tip of the nozzle of the toner supply device into the sub
rotary developing unit through the slit on the elastic seal in order for
the toner supply device to supply toner into the sub rotary developing
unit, and pulls the tip away when toner is properly supplied.