PURPOSE: To obtain a connection structure which requires only a small space and which can be easily constructed, by fixing a steel pipe provided with external protrusions to the head of a foundation pile and fitting a large diameter steel pipe provided with internal protrusions on the above pipe and placing concrete in the inside to unify them.

CONSTITUTION: A steel pipe 2 provided with protrusions 4 at the external face is fixed to the head of a foundation pile made of a driving steel pipe pile, a soil cement composite steel pipe pile, a cast-in-place steel pipe concrete pile, etc. and a large diameter steel pipe 1 provided with protrusions 4 at the internal face is fitted. The protrusions 4 are formed as spirals or intermittent stripes to place concrete without voids and unify them more securely. Concrete 3 is placed in the inside of steel pipes 1, 2 to connect together and a footing or a column base is connected through the steel pipe 1. This structure dispenses with auxiliary metal fittings for connection of a foundation slab or a steel cage or the like. In this way, a connection structure which is easily constructed can be obtained.