PURPOSE: To facilitate automatic manufacture by disposing outer-circumferential side plates on the outer circumference of a spiral staircase section and vertically forming each end connection of the outer-circumferential side plates so as to coincide with the module of a building under the state, in which a spiral staircase is assembled.

CONSTITUTION: A spiral staircase section 1 is formed of a staircase section 11 continuously connected to a lower straight staircase section 2 and a stair landing section 12 continuously connecting the staircase section 11 and an upper straight staircase section 3. In the staircase section 11 of the spiral staircase section 1, two strips of horizontal grooves 131A, 131B are formed to an outer- circumferential side plate member 131 on the lower side, and one end of a tread 111A at a lowermost step is engaged into the lower groove 131A and a tread 111B at an intermediate step into the upper horizontal groove 131B. An end connection on the lower step side of the outer-circumferential side plate member 131 on the lower side is formed vertically along one end of the treading-in plate 112B of the tread at a lower side under the state, in which the spiral staircase is assembled. Accordingly, the spiral staircase can be unitized on the basis of height.