PURPOSE: To eliminate a height difference between two adjacent bed boards during extension and retraction, to satisfy the connection therebetween and to facilitate the disconnection therebetween, by dividing each bed board into two front and rear parts so that an extendable one thereof covers a center bed board in an advance direction.

CONSTITUTION: On a slow speed conveyer 4, two separate front and rear extendable parts 2, 3 are laid on a core 1 which is therefore covered therewith. When they come onto a high speed conveyer 5, an attachment 8 thereof enters through a cut-out 7 in the core l, and is caught by the lower part of a front panel of the extendable part 3 which is therefore pulled out from the bed board 1. Then protrusions projected from the core 1 are caught by a groove 9 in the expandable part 3 so that the core 1 is pulled out, and the expandable part 2 is pulled so that the following bed board is extended in order to enhance the speed of the movable sidewalk. The upper surface of the core 1 is covered with expandable parts 2, 3 in its entirety, and beads are formed allower the surfaces thereof, in parallel with the advancing direction which receive similar protrusions formed on the core l. Connectors 2', 3' which are rotatably engaged with the adjacent bed board are provided to the side wall of the expandable parts 2, 3.