PURPOSE: To take out the extra-banking part of concrete rapidly with low noise level by a method in which a strength-lowering substance is injected into the extra-banking concrete part above the top end of an existing pile, and after a given strength is reached in the concrete pile, the extrabanking concrete part is cut off.

CONSTITUTION: An injection pipe 3 is penetrated to the lowermost layer of an extra-banking concrete C' and then turned while injecting a bentonite liquid as a strength-lowering substance under high pressures into the pipe 3. By pulling up the pipe 3 while being turned, the bentonite liquid is mixed with the whole of the extra-banking concrete C'. After the concrete pile C is hardened until a given strength is reached, the ground surrounding the pile is excavated to form an excavated pit and the upper extra-banking concrete part C' from a set top end P of the concrete pile C is cut off by a rock drill, etc.