PURPOSE: To produce a sealed filling bag capable of being unpacked laterally by using the film having one face alone heat sealable and improving slightly the conventional automatic bag forming machine.

CONSTITUTION: The film having the lateral elongation rate much larger than the longitudinal elongation rate at least on one layers is used, a seam joint is formed between longitudinal heat sealing parts 2 and, at not less than one place thereof, an unpacking piece 4 is provided therebetween extending to such a length as to permit its end to be held with the fingers. One film of the longitudinal heat sealing parts is provided with a projecting part extending sufficiently long to be held with the fingers and left unsealed and the other film thereof is heat sealed to a side edge line and, along this line, a group of densely collected fine damage traces is provided.