PURPOSE: To completely control locking/unlocking time for driving plates by providing locking members freely movable to locking and unlocking positions and a driving means to drive the locking members.

CONSTITUTION: When an adjacent driving plate in lock state is forwarded, the locking member is also forwarded by engaging with an arresting member. When forwarding of the driving plate is prevented by a preventing member, an adjacent driving plate is unlocked at the same time. One driving plate is fixed and the other driving plate is continuously forwarded. In two adjacent driving plates in unlock state, when one is backwarded and brought into contact with the other, moving of the locking member is started and the locking member is engaged with the arresting member fitted to the adjacent driving plate to lock the adjacent driving plate. Thus, the driving plate is always positioned and locking and unlocking are timely controlled to prevent a malfunction.