PURPOSE: To form a high-strength cast-in-place pile by a method in which a steel tube is penetrated into a pit under the ground, an inner tube is inserted into the steel tube, concrete is placed into the tube, and when the concrete is not hardened, only the steel tube is pulled out.

CONSTITUTION: A longer steel tube 4 than the length of a pit 3 under the ground is penetrated into the pit 3, a cylinder 5 of the same length as the tube 4 is inserted into the tube 4, and a reinforcing bar cage 6 is set inside the cylinder 5. Concrete 8 is then placed into the cylinder 5, and before the concrete 8 is hardened, the tube 4 is pulled up by leaving the cylinder 5 under the ground. The wall soil of the pit 3 can thus be isolated from groundwater or unhardened concrete by the cylinder 5, and the inflow of unhardened concrete and groundwater and also the inflow of cement into the underground equifer can also be prevented, permitting high-strength cast-in-place piles to be formed.