PURPOSE: To prevent the settlement and floatation of a buried pipe by boring a plurality of pairs of boring pits up to a hard ground on both sides of the existing buried pipe, executing a let grout to the pits and around the buried pipe and inserting steel bars into the pits.

CONSTITUTION: A plurality of boring pits 11 are bored up to a hard ground 5 while being oppositely faced on both sides of a buried pipe 1 such as a sewerage pipe. The boring pits 11 are impregnated with cement milk while a jet grout is executed to the periphery of the buried pipe 1, and a massive improved body 25 is formed while the buried pipe 1 is borne by a columnar improved body 23. Steel bars 19 are inserted before cement milk is cured, and the buried pipe 1 is reinforced. The steel bars 19 may also be inserted into the pits 11 before the impregnation of cement milk. Even when a liquefaction phenomenon due to land subsidence and an earthquake is generated, the buried pipe 1 is not subject to the effect. Accordingly, the floatation and settlement of the buried pipe can be prevented.