PURPOSE: To reduce works along with laying a cable and enhance the safety of the works, by connecting underground conduit line sections with a flexible pipe, and by travelling an automatic travelling device to which the tip of the cable is connected, into the pipe, to lay the cable.

CONSTITUTION: When a cable 4 is laid on an underground conduit line 2 formed penetrated through a man-hole 1, then a space between the conduit line ports 1a and 1b of the man-hole 1 is continuously connected with a flexible pipe 13 first, and together with the underground conduit line 2, a continuous conduit line without a step difference is formed. In the meantime, an automatic travelling device 14 which can be travelled into the conduit line is prepared for, and on the device 14, the tip of the cable 4 to be laid is fitted. Then, a conductor in the cable 4 is used, and power is fed to the automatic travelling device 14, and at the same time, the automatic travelling device 14 is travelled into the conduit lines 2, 13, and the cable 4 is laid.