A conduit comprised of double-wall pipes A is laid in the ground by the pipe-jacking method. Each of double-wall pipes A comprises inner pipe 1 and outer pipe 2 and is provided with sheath pipe 4 between inner pipe 1 and outer pipe 2 and with check valve 7. Winch 11 is mounted in tunneling machine 6. Wire 12 wound on winch 11 is inserted into sheath pipe 4 and the forward end of wire 12 is sent to start vertical shaft C through sheath pipe 4, and connected with cleaning member 24, injecting member F, and supply pipe 14. Cleaning member 21, injecting member F, and supply pipe 14 are moved in sheath pipes 4 by driving winch 7. When injecting member F is opposed to check valve 7, packing means 19, 20 are inflated to be brought into contact with the inside of sheath pipe 4 by which injecting member F is fixed to sheath pipe 4. Lubricant or back-filling material is sent to a space between packing means in sheath pipe 3, and injected through check valve 7 into space 9 between the outside of outer pipe 2 and the ground.