series of regularly spaced holes on the side walls thereof into which mounting bolts 3 and 4 are inserted as particularly shown in fig2 a stair step member 5 includes a horizontal part or tread 5a and a vertical part or riser 5b the lower end portion of said tread 5a has a wound portion 5c through which a mounting bolt 4 is inserted and the upper end portion of said riser 5b has a wound portion 5d through which a mounting bolt 3 is inserted the stair step member 5 is thus supported and fastened by the mounting bolts 3 and 4 to the stringers 1 and 2 all other step members are assembled literally step by step in the same manner in a second preferred embodiment of the invention shown in fig3 two higher and lower level step members 5 and 6 are assembled together such that the upper wound portion 6d of the lower level step member 6 receives and is supported by the same mounting bolt 3 through the lower wound portion 5c of the higher level step member 5 fig4 indicates a third preferred embodiment of the invention in which a two level step member 7 is shown and includes a vertical riser 7b and lower and higher level horizontal treads 7a and 7c each of said treads 7a and 7c having a step of a substantially one half smaller length than the height of said riser 7b the lower level tread 7a has a downwardly bent portion 7d whose end is in the form of a wound portion 7f through.