To accomplish cost reduction and facilitation of assembling adjustment work.

An endless roller chain 13 is laid on rotation bodies 12A, 12B turnably supported around a left/right direction axis in front and rear sides in a longitudinal direction of a fixed fork 11 and is driven by a drive device 4, an intermediate stage dog 27 engaged with the roller chain 13 is fixed to an intermediate fork 21. Further, in front and rear sides in a longitudinal direction of the intermediate fork 21, the endless roller chain 23 is laid on rotation bodies 22A, 22B turnably supported around the left/right direction axis, a lower stage dog 17 engaged with the roller chain 23 is fixed to the fixed fork and an upper dog 33 engaged with the roller chain 23 is fixed to a distal end fork 31.

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