An insert with a light weight per unit length, which is securely separable to obtain insert pieces, a method for producing such an insert, and a method for producing long products using such an insert. The insert includes a large number of insert pieces and joining parts formed by the application of a rolling operation and a tension force, and has a ladder-shaped configuration. Slits are defined between adjacent insert pieces, and include first slits, each having a large width in a longitudinal direction of the insert, and second slits, each having a width smaller than that of the first slits so as to be alternately formed. The joining parts include first joining parts, each having a small width in a width direction of the insert, and second joining parts, each having a width larger than that of the first joining parts so as to be alternately formed. The width ratio of the insert pieces, the first slits and the second slits in the longitudinal direction is determined to 100:(65˜104):(24˜53), whereas the width ratio of the first joining parts to the second joining parts in the width direction is determined to 20:(28˜33).