PURPOSE: To enable the width of a base plate transfer section to be controlled while the base plate transfer section is held to be light, by a structure wherein a movable guide rail structure is made to follow the movement of a movable guide beam structure, and a brake mechanism is provided which prevents the false movement of the movable rail structure.

CONSTITUTION: When a guide rail structure 52 is stuck between a loading bridge 5 and an un-loading bridge 7, a movable guide rail structure 53 is connected to at least one movable beam structure 41. When the movable beam structure 41 is moved by a space changing power source 41, the movable guide rail structure 53 follows the movement of the structure 41 to move. At this time, the movable guide rail structure 53 is released from a brake mechanism. When the movable beam structure 41 is stopped, correspondingly, the movable guide rail structure 53 is also positioned to a new position. At this time, the brake mechanism is again operated to lock the movable guide rail structure 53. When the guide rail structure 52 goes downward from the position between both the bridges, connection between movable rail structure 53 and the movable beam structure 41 is released. Accordingly, the base plate transfer width of the base plate positioning mechanism can be controlled, following the control of the base plate transfer width, and moreover the weight of the base plate positioning mechanism is vainly increased.