PURPOSE: To secure safety by the surrounding by a safe cover and to facilitate getting-on/off by allowing a getting-on/off port to be set always at the position optimum for getting-on/off.

CONSTITUTION: A staircase 6 is installed in spiral form around a longitudinal shaft 5. A vertical elevation means for vertically raising and lowering the staircase 6 through the vertical movement around the longitudinal shaft 5 is installed. A cylindrical safe cover 8 is installed in turnable manner on the periphery of a staircase 2, surrounding the staircase 6. A getting-on/off port 9 is formed in correspondence with each floor on the safe cover 8. A safe cover driving means for driving the safe cover 8 in synchronization with the rise and lowering of the staircase 6 is installed. The getting-on/off port 9 is allowed to correspond to the getting-on/off position by turning the safe cover 8 according to the rise and lowering of the staircase 6.