PURPOSE: To lay a new element through a conduit in which an existing element has been laid by so specifying the maximum outer diameter of a wire strand as not to intrude or engage the strand into or with a gap between the existing element and the conduit.

CONSTITUTION: A large-diameter part 21 having a maximum outer diameter (a) for satisfying an equation (where a is the maximum outer diameter of a line body 3, D0 is the inner diameter of a conduit 2, D1 is the outer diameter of an existing element 1, θ is an angle formed between a straight line for connecting the center O of the conduit 2 to the center A of the element 1 and a vertical line passing the center O of the conduit 21 at a longitudinal suitable position of the line body 3. Thus, intrusion of the line body 3 into the gap 11 of the conduit 2 to the element 1 is prevented, and frictions of three elements are not increased. Further, it is desirable always to form the part 21 to dispose at least once at the time of inserting the line body 3 in or laying a new element through a conduit.