To efficiently insert a steel pile into the ground by substantially I-shaping an axially vertical cross section of a main body in the same as a sectional shape of a train rail, and improve the steel pile in a viewpoint of effective resource utilization by using a used train rail as the steel pile.

An axially vertical cross section of a main body of a steel pile is formed in the same as or substantially I-shaped similarly to a sectional shape of a train rail. In particular, the main body is formed from a used train rail and used as a soil improvement pile, a foundation pile, a retaining wall pile, a connecting wall pile, an earth retaining pile or the like.