PURPOSE: To provide a measure to prevent an operator from carelessly entering an inspection passage mounted between two shelves at a low cost.

CONSTITUTION: Elevating ladders 22, 23 which are commonly used as a door to open/close the entrance/exit are pivotably supported in an openable/closable manner at the entrance/exit at the end part of the respective passages A-C of a plurality of stages in the vertical direction which are partitioned by inspection passages 2, 3, and a locking means to lock the respective ladders 22, 23 for elevation at the closing position of the passage between the shelves is continuously arranged.