To improve workability and functions by laying a curb-shaped frame unit stepwise on a slope face and fixing it in a specified state and thereafter, filling sand and soil or the like into the frame body.

Vertical wooden bars 2 and cross wooden pieces 3 are connected to each other through a pair of incisions formed in the intersection to form a curb-shaped frame body and the connection parts are fixed by bolts 4 to constitute a unit for constructing a wooden stair and further corrosion-preventive and termite-preventive treatments are applied. On installing it, the unit 1 fabricated in advance in a factory is laid on a slope face 9 stepwise and then, arbitrary positions where mutual units 1 are jointed to each other by deformed steel bar-shaped jointing means 7 and the unit is fixed to the sloped face 9. Subsequently, a filler such as sand and soil or the like is filled in the frame body of the unit 1 and it is sufficiently compacted to form a tread face. In this way, the workability can be greatly improved and run-off of soil due to rainwater or the like can be prevented. If thinnings are utilized, forest resources can be effectively used and further, durability can be improved by using corrosion- preventive and termite-preventive wooden material.