PURPOSE: To easily connect both pipes by providing an engagement slit opening upward on an upper end part side wall of a first single pipe and an engagement projection free to engage with the engagement slit on a lower end part of a second single pipe.

CONSTITUTION: A plural number of engagement slits 11 are circumferentially provided in the circumferential direction of an upper end part of a first single pipe 10, while a fitting cylinder 31 is previously fit, projected and welded on the inner peripheral surface of a lower end part of a second single pipe 30, and engagement projections 32 are respectively provided circumferentially at positions corresponding to the engagement slits 11 of the first single pipe 10. After one of unit steel pipe piles having such coupler structure is driven into the foundation, a fitting part of the other unit steel pipe pile and an end part of the one unit steel pipe pile are fit to each other so that an engagement projection of the other unit steel pipe pile is inserted to a bottom part of an insertion groove from an end part of the insertion groove of the engagement slit of the one unit steel pipe pile. The engagement projection of the other unit steel pipe pile is engaged with the engagement groove one unit steel pipe pile by rotating the other unit steel pipe pile or the one unit steel pipe pile in the circumferential direction.