PURPOSE:To reduce the load necessary for driving and to miniaturize and light- weighing the driving source by pressing to a catching member a 2nd guide post unit by the pressing member displacing according to the cam which is provided on the rotary member to rotate for moving a 1st guide post unit. CONSTITUTION:The structure to press for a catch 42 the 2nd guide post unit 20 by the cam 41 rotating by the cam 46 provided on a loading ring 1 is made and the pressing force for the catch 42 is obtd. by a spring. The driving load of the loading ring 1 and driven ring 6 does not become large so much even if the energizing force of this spring is made large enough. In this way the loading motor can be miniaturized and light-weighed and the sure catching of the guide post unit is enabled as well.
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