PURPOSE: To increase the support strength of a pile by forming a cement layer on the circumference of an upper pile relatively smaller in the diameter with a cement milk injected into a clearance between the circumference of the upper pile and the subsoil when driving the foundation pile joining a pile of varied diameter to be driven into a poor subsoil or the like.

CONSTITUTION: When driving a pile assembly, first, a lower pipe 1 large in the diameter with a smaller diameter part formed at the upper end thereof is driven by striking or the like. Subsequently, a small diameter medium pile joins the upper end of the lower pile 1 with anasphalt layer 2 applied on the circumference thereof and similarly, driven with the lower pile by striking or the like until the lower end of the lower pile 1 reaches a foundation bed 8 of the subsoil 7. Then, a sheath 10 is inserted through the top of the upper pile 4 and a flange 15 at the lower end thereof is loosely fitted onto the circumference of the upper pile 4. Thereafter, the sheath 10 is withdrawn while a cement milk 16 is being injected from an injection tube 11 mounted on the inner wall thereof. In this manner, a cement layer 17 is formed on the circumference of the upper pile 4 to complete the work.