To completely autamate a manufdcturing process for a truss member used to synthetic slab.

A manufacturing device for a member is composed of: an upper cutter, a lattice automatic forming machine, and a truss member automatic forming machine 17. The upper member cutter cuts a long-size wire rod to form upper member, having a given length, in order. The lattice member automatic forming machine form a lattice 76 wherein another wire rod is folded alternately to provide first and second folding parts alternately. The truss member automatic forming machine 17 is equipped with: a welding unit 133; arranged in material feeding line downstream sides of the upper member cutter and the lattice member automatic forming machine, to weldingly join the upper member 40 and the second folding part of the lattice member 76; and a folding unit 134 for foldingly forming alternately the first folding part of the lattice member 76 in an orthogonal direction to a carrying line.