To provide a manufacturing method for a gusseted flat bag for a bifurcated triangular lug wherein left and right side surface parts and an undersurface part are formed at an end part as a gusset body between front and rear surface parts.

A twofold gusset continuous body F3 is superposed on the first continuous packaging material F1 in a main packaging material isolation area separating first and second continuous main packaging materials F1 and F2 to transfer them and the second continuous packaging material F2 is superposed in a packaging material uniting area on the downstream side. A gusset forming folding body F4 equipped with the bifurcated triangular lugs F4 and F4', wherein one flap F4a extends flatly and other flap F4a' is opened by 90° or more, is superposed on and temporarily fastened to the first continuous main packaging material on the upstream side of the position where the gusset continuous body is superposed on the first continuous main packaging material. The flatly extended flap is held between the gusset continuous body and the first main packaging material while the other flap is laid down on the gusset continuous body to be inserted in the gap between the gusset continuous body and the second packaging material and the flaps are laterally and vertically heat-sealed in the packaging uniting area on the downstream side to obtain the gusseted flat bag P1 being a continuous laminate.