A worm gear is provided on a rotating shaft of an AF motor. A first
supporting cylinder includes a gear portion and a receiving portion. A
wheel gear, which is in engagement with the worm gear, is formed on an
outer surface of the gear portion. A slit is formed at the receiving
portion. A second supporting cylinder includes a supported portion which
is placed in the receiving portion of the first supporting cylinder, and
a receiving portion which supports an AF coupler. A gear is in engagement
with a portion which connects the supported portion and the receiving
portion of the 2nd supporting cylinder. A clutch spring is wound around
the supported portion, being in contact with the outer surface of the
supported portion. Both ends of the clutch spring cross each other and
are in engagement with a pair of walls, of the slit, which are parallel
to an axis of the receiving portion.