PROBLEM TO BE SOLVED: To provide a spacer for work execution of a stair treadboard allowing treadboard fitting work to a side plate to be performed easily and accurately at the time of work execution of stairs and maintaining a firmly fixed state over a long period of time. SOLUTION: The end part of a treadboard 3 is inserted in an insert groove 2 provided at a side plate 1, through a spacer member 4, and fixed. The spacer member 4 is formed of a vertical pair of spacer pieces 4A, 4B. The opposed faces of these vertical spacer pieces 4A, 4B are formed into inclined faces 6a, 6b provided with continuous serrated engaging protrusions 7, 8 engaged with each other, and provided with protrusion streaks 9 and grooves 10 engaged with each other so as to guide the upper spacer piece 4A in the insert direction. The upper spacer piece 4A is shifted in the cross direction from the lower spacer 4B and placed in a state of being protruded to the inside from the opening end of the insert groove 2, and the upper spacer piece 4A is driven into the insert groove 2 with a hammer or the like. The upper spacer piece 4A is therefore gradually moved upward to force-fit and fix the end part of the treadboard 3 placed on the upper spacer piece 4A, to the upper face of the insert groove 2.