PURPOSE: To facilitate the locking/releasing operation of the lock from both the face side and the rear side, and the confirmation of the locked condition by arranging a locking device below the loading surface when a base panel body is horizontally collapsed, and operating the operating part from the loading surface side.

CONSTITUTION: A back panel body 10 and two side panel bodies 20 are arranged in a portal shape when plan-viewed, and a base panel body 33 is fitted to the lower part of the panel body 10 in an erectable/collapsible manner at the proximal end, and a locking device 40 having an operating part acting on the other panel 20 side is provided thereto. The locking device 40 is arranged below the loading surface 39 when the base panel body 33 is horizontally collapsed, and the operating part 44a is operable from the side of the loading surface 39. As a result, the locking device 40 can be locked or released by operating the operating part 44a from the side of the loading surface 39 when the base panel body 33 is horizontally collapsed, and the locked condition can be confirmed by looking the operating part 44a from the side of the loading surface 39.