To simply, economically, and continuously extend a material three- dimensionally by a method wherein in the case of a linear, a rod-form, and a belt-like material, two or four materials making contact with each other are tightly bound at equal intervals alternately by using two kinds of tight binding patterns and thereafter, uniform extension is effected in two directions vertically to the section of a collection bundle.

In a reinforcing bar bundle, two or four reinforcing bars making contact with each other are tightly bound at equal intervals alternately according to two kinds of tight binding patterns A and B and each outer tight bonding point is grasped by a expander and by expanding in two directions vertically to the section of the reinforcing bar, a continuously three-dimensionally expanding reinforcing bar netting is produced. This constitution effects continuous, simple, and economical three-dimensional expansion of materials heretofore considered to be complicated and uneconomical.