PURPOSE: To make it possible to draw an expansion floor in the bottom of an upper layer stair seat in a state to mount an auxiliary step installed on the expansion floor and, at the same time, to make it possible to assemble or fold the auxiliary step with quick and simple operation.

CONSTITUTION: A pair of front and rear bearing boards 11 and 13 pivoted on an expansion floor 3 are raised vertically, and a tread 15 pivoted on the upper end thereof is lifted horizontally. After that, both side plates 18 are raised at a vertical position, and the bottom of the tread 15 is borne on the upper edge thereof. According to the constitution, a step device is built up to make it in a usable state, and when both side plates 18 is fallen down to the surface of the expansion floor 3, a brake of the tread 15 is released, and the step device can be folded in a flat and low shape capable of storing it in a gap between the upper layer seat 1 and expansion floor 3.