To reinforce a foundation using piles excellent in the strength of their connections, through a simple operation not requiring operators special for welding.

A new footing part 14 is added to the footing part 8 of the pier 6 of a speedway 2, and reinforcement is achieved using steel pipe piles 16. To construct the steel pipe piles 16, holes 20 are first excavated vertically by an excavator. Also, a number of short steel pipes 18 are prepared. The steel pipe 18 has an externally threaded part formed at one longitudinal end and has an internally threaded part at the other. In a vertically narrow space S below the base course 4, the externally and internally threaded parts at the ends of the steel pipes 18 are engaged with each other, and as the steel pipes 18 are sequentially extended they are inserted into the holes 20, with the desired length achieved by the number of steel pipes 18 coupled together by the threads, so that the steel pipe piles 16 are constructed.