PURPOSE: To make an actuator for a position reference member unnecessary, by a structure wherein a stopper on the side of a loading bridge and the position reference member on the side of a base plate positioning mechanism are simultaneously advance or retreated by means of an actuator on the side of the loading bridge.

CONSTITUTION: A loading bridge 5 and an un-loading bridge 7 are left in a space each other, in which a base plate positioning mechanism 3 is stuck, to be disposed at a fixed distance. When the base plate positioning mechanism 3 is located in a line with respect to the loading bridge 5, the movement of an advance/retreat actuator makes a stopper 16 to retreat and the motive power for retreating the stopper 16 to be transmitted from an output member to a power transmitting member 127. Moreover, a position reference member 120 is also moved to the retreat position thereof, so that a base plate 1 can pass through freely the loading bridge 5 as well as the base plate positioning mechanism 3. Accordingly, an advance and retreat movement can be performed without an actuator included in the position reference member 120 being operated.